Introduction

Independent public toilets are essential for the functioning of modern cities, especially in densely populated areas where they serve as critical infrastructure1. These facilities are integral to daily life and tourism, particularly in locations lacking municipal pipeline connections2,3. For the current urban growth, environmental degradation, population explosion, lack of resources, and other challenges that must be addressed, the current number, distribution, and humanized toilet facilities are insufficient to fulfill the growing needs of users4. Despite urban growth, current toilet numbers and distribution remain inadequate, with only 3.29 public toilets per 10,000 people in China by the end of 2021, unevenly distributed5. Traditional flush toilets, reliant on extensive energy, water, and sewage networks, often face operational challenges and environmental pollution risks due to maintenance neglect6,7,8. Given these challenges, a key research question arises: How can energy-autonomous technology (EAT) be applied to improve the energy efficiency and climatic adaptability of independent public toilets, particularly under different climatic conditions? To address this, we explore the potential of EAT in reducing environmental impact while ensuring sustainable operation. Independent public toilets can utilize renewable energy sources, aligning with China’s strategy to reduce CO2 emissions and support sustainable development goals9. By investigating the performance of EAT across various climates, this study aims to provide practical solutions for enhancing public health infrastructure in water-scarce or underdeveloped areas.

Public toilets serve users who are temporarily occupying the space. In cold climates, space heating is typically controlled between 10 and 14 °C, with 12 °C being common, to prevent indoor water from freezing10,11,12. In southern regions, lower temperatures (around 5 °C) during non-use periods suffice. Room temperature should generally not exceed 28 °C for cooling requirements13, but in northern regions, cooling is often unnecessary due to outdoor temperature fluctuations averaging 1.5 °C10,14. The fan must also make sure that air exchange and ventilation occur between toilets at least 5/h15,16,17. From a comfort and safety standpoint, lighting fixtures for standalone public restrooms are an absolute necessity, and interior lighting must satisfy the standard of ≥ 50 lx illumination15. Cold water in most of China averages 10 ~ 20 °C and needs to be heated to 45 °C at distribution points18. Hot water for washing is not necessary for autonomous public toilet operation. Energy consumption for specialized equipment, such as sanitary ware controls, intelligent monitoring devices, and excreta treatment systems, depends on usage duration and performance19.

Conventional public toilets face energy supply challenges, including reliance on outdated energy sources, excessive consumption, and environmental damage20. To operate sustainably, independent public toilets must adopt innovative energy solutions21. Autonomous technology aims to reduce building-related environmental pollution by utilizing clean energy, targeting zero-energy buildings22,23,24. Studies have evaluated the impact of Building Energy Management Systems (BEMS), Phase Change Materials (PCM), Solar PV, and Ground Heat Pumps (GHP) on energy consumption, demonstrating that photovoltaic panels can significantly meet electricity needs25,26. For instance, Noorollahi et al.27 used solar collectors and thermal storage with particle swarm optimization for optimal energy self-sustainability. Advances in autonomous technology have spurred interest in zero-energy, sustainable, and green buildings, leading to new design concepts and assessment systems28,29,30. Energy Internet of Things technology has been used in autonomous buildings to collect energy consumption data for analysis and reduction of building energy consumption30,31. Rahim Zahedi et al. used green roof technology to effectively reduce heat loss from roofs and walls, saving 54.95% in winter and 76.11% in summer32.

The use of energy-autonomous technology (EAT) in independent public toilets means that they rely as little as possible on external power or heat supply and that they reasonably select appropriate renewable energy sources to replace external energy supply based on their energy demand, resulting in a balance between energy supply and demand33,34. To achieve higher energy consumption efficiency, choose the right site or building space form based on renewable energy resource requirements. Excreta treatment needs energy consumption, and independent public toilets that use air-cooled solar photovoltaic thermal (PV/T) technology for energy delivery can provide both power and hot air35. Electricity powers the motorized equipment throughout the treatment system, and hot air provides the thermal energy essential for the excreta treatment process36,37,38. Active air-cooled PV/T technologies that directly supply energy to freestanding public toilets must consider the demand for energy for excreta disposal in toilets, so the system’s heat production performance and the consistency with which it operates in various regions must be investigated further39. There is still a dearth of adaptive research into the use of EAT in autonomous public toilets, and the system’s climate adaptation must be considered40. Based on the current state of the art, this study investigates the energy performance and climate adaptation of independent public toilets using energy-autonomous technology (EAT), which can solve the problem of energy balance in public toilet operation, particularly excreta treatment. To evaluate the adaptability of this technology under various climatic circumstances, the study used the DesignBuilder building energy simulation program to simulate and analyze energy consumption. This study addresses a gap in previous research on the use of EAT in independent public toilets by offering in-depth modeling and analysis of the use of energy-autonomous technology (EAT) in independent public toilets using the DesignBuilder building energy simulation program. This is the first study to comprehensively examine the performance of EAT technology in public toilets under various climatic circumstances, demonstrating the significant impacts of light intensity and temperature on EAT system efficacy. This study also investigates the specific application strategies of EAT technology in various geographic environments, and the findings are significant for promoting the long-term development of independent public toilets, increasing energy efficiency, protecting the environment, improving infrastructure development, and promoting practical applications. A more sustainable, efficient, and environmentally friendly public toilet system can be built through extensive research and promotion of EAT technology in independent toilets, particularly in areas where water resources are scarce or infrastructure is underdeveloped, resulting in clean and hygienic toileting conditions for residents.

The paper is structured as follows. The following section designs the model of EAT for public toilets, determines the simulation parameters, and selects different climate zones. Section “Results” is to analyze the results of the energy simulation, and the performance of the technology in terms of heating, emission reduction, and climate adaptation. Section “Conclusion” concludes the significance of the work in this paper.

Methodology

This study aims to explore the feasibility and effectiveness of applying energy-autonomous technology (EAT) in independent public toilets, with a particular focus on their energy efficiency and climatic adaptability under different climatic conditions. To this end, we used a park scenario as the context of use and constructed a model of an independent public toilet that includes PV/T technology for energy supply and aerobic bio-composting for on-site excreta treatment. To evaluate the performance of EAT, we conducted building energy consumption simulations using DesignBuilder software, which works alongside the EnergyPlus calculation engine to dynamically simulate building energy consumption. The specific steps included: selecting the building type and template, setting building elements (such as walls, windows, doors, etc.), and adding parameters for materials, equipment, and personnel activities within the rooms. By simulating the energy consumption of independent public toilet models in five different climatic zones, we assessed the self-sufficiency of the model’s energy supply and its performance under various climatic conditions. This process validated the potential of EAT to enhance the sustainability and environmental adaptability of public toilets.

Energy autonomous technology for public toilets

The autonomous toilet energy supply technology primarily employs photovoltaic panels to realize solar photovoltaic utilization, in which solar energy is transformed into electricity and stored in a battery before being given to motors, pumps, fans, lighting, and other electrical equipment. The use of fins beneath photovoltaic panels can better cool the photovoltaic panels so that the efficiency of the conversion of thermal energy is further improved to enhance the efficiency of power generation27, and at the same time to realize the use of thermal energy, but also to ensure that the structure of the roof is sturdy and stable. The thermal energy gathered by the device is primarily used as the heat source in the composting environment21. The schematic diagram of the technical model is shown in Fig. 1.

The roof PV panels of the toilet model in this study have an angle of around 26.565°, making them ideal for gathering solar energy in most locations of China. The entire PV device measures 1.36 m broad and 3.3 m long, covering an area of approximately 4.5 m2. Thermal collector fins, thermal collector baffles, and a top plate are positioned in order beneath the photovoltaic panels. An air inlet is located on the low side of the rear of the PV panel, and the system can use thermal pressure ventilation for improved airflow. The air completes the heat collection process by moving from the bottom to the top and collecting the hot air through the top air collector port. The hot air runner then uses the cavity in the middle of the hollow roof panel and the back wall panel to carry the hot air to the bottom compost bin. In the early phases of composting, when the initial ambient temperature hits around 25 °C, the microbial activity will frequently be enhanced because the compost heap includes a significant amount of organic matter. It is essential to boost the compost heap’s temperature to over 55 °C. When the organic content and microbial activity are reduced, the compost heap’s temperature gradually decreases and enters the subsidiary stage, eventually approaching the ambient temperature41. In the future, the PV panels can be enlarged around the toilet module to meet the specific needs of the application or the angle of the roof PV panels can be altered to account for latitude and seasonal fluctuations in the application location.

Fig. 1
figure 1

Schematic diagram of EAT for independent public toilets.

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Simulation modeling and parameterization

In this study, DesignBuilder software was utilized to build a physical model according to the dimensions of the scheme, with a total area of 4.9 m2, of which the area of the room to be supplied and cooled is 1.74 m2, as shown in Fig. 2. First, select the building type and template, enter the relevant parameters and draw the model. Then, select architectural elements such as walls, windows, doors, etc. and set their parameters such as width, height, material, etc. Finally, add and edit architectural elements in each room, and then set up a variety of materials, equipment and personnel activity conditions. The parameters of the model’s envelope setup are shown in Table 1. Meteorological data from multiple locations are supplied individually during the simulation, allowing for dynamic simulation of cases throughout the year. The internal disturbance variables and equipment details of the physical model were determined based on the operation of the independent public toilet. Personnel activity was limited to 06:00 to 22:00, with an average of one person entering and using the bathroom every 9.6 min, around 100 persons using the toilet in a single day, and no one using the toilet at other times42,43. Lamps are programmed to run 10 W energy-saving lamps for 6 h in a single day during adverse lighting conditions. Exhaust fan according to the main and auxiliary fan, a single day full power operation 8 h, the rest of the time according to the low power operation settings, daily power consumption of 0.312 kWh. Pumps are programmed to operate intermittently for 1.5 h per day, with a daily power usage of 0.375 kWh44,45; pumping, mixing, and other equipment, a total of 4 intermittent operations, a total of one hour of settings every day, and a daily power usage of 0.36 kWh. To ensure that the water pipe does not freeze in winter, in the simulation setting, the indoor temperature during the unoccupied period at night is no less than 5 °C, while the daytime operation ensures that the indoor temperature is no lower than 12 °C as far as practicable11.

Fig. 2
figure 2

(a) Modeling interface of DesignBuilder software. (b) Material interface of DesignBuilder software.

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Table 1 Simulation model envelope materials and parameters46,47.
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Location selection for climate adaptation analysis

This study examines the performance of EAT for independent public toilets across different climatic zones, taking into account its stability. A total of 5 locations belonging to 5 different thermal zones in China were selected for the comparison, taking into account the differences in temperature, irradiance, and altitude of the 5 locations. The 5 chosen locations (Fig. 3) are Tianjin (North China-cold region-2B), Xining (Northwest China-cold region-1 C), Nanning (South China-hot summer and warm winter region-4B), Hangzhou (East China-hot summer and cold winter region-3 A), and Lhasa (high altitude-cold region-2 A)11.

Fig. 3
figure 3

Location and global horizontal irradiance of the five cities.

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The 5 locations are located in various climatic zones in China, and their climatic characteristics reflect both moderate and intense solar irradiation, as well as high and low-altitude areas. The locations were chosen to accommodate for geographical and climatic heterogeneity, as well as to give a contrast that may be utilized as a typical location to compare the performance of EAT in diverse geographical situations. Typical meteorological yearly weather data for the five locations were obtained from the National Solar Radiation Database (NSRDB)48.

Results

Energy consumption simulation

The simulation results of the model’s year-round building energy consumption for 5 cases are shown in Table 2. The toilets are heated with graphene electric heating, and the simulation results for all cases show that the minimum indoor temperature of the toilets is not less than 12 °C in winter during operating hours, and not less than 5 °C at night when the toilets are closed. If refrigeration energy consumption is not taken into account, the 5 cases are ranked in descending order of overall energy consumption: Xining, Tianjin, Lhasa, Hangzhou, and Nanning. Because the quantity of refrigeration energy consumed in each of the 5 cases is tiny, Xining has the majority of total energy consumption. Although Nanning has the highest cooling energy use, its total energy consumption remains the lowest. As a result, the locations with high heating energy consumption had the greatest overall energy consumption, with Xining City, the coldest region, having the highest total energy consumption of 1187.67 kWh.

Table 2 Comparison of simulation results for energy consumption in 5 locations.
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A comparison of the technical performance of the 5 locations is shown in Table 3. The five locations are ranked according to PV capacity as Lhasa, Xining, Tianjin, Nanning, and Hangzhou, and it is clear that locations with higher levels of radiation have certain advantages. The thermal comfort power consumption rate might reflect the overall amount of power generated in the thermal comfort function while the toilet is in operation. The table shows that the thermal comfort power consumption rate is close to 50–60% in Tianjin and Xining, while the rest of the cases account for less than 20%. Based on the descending order of the thermal comfort energy consumption rates of the five cases, the results are Xining, Tianjin, Hangzhou, Lhasa, and Nanning. Because the energy consumption of various electrical equipment in toilets is virtually identical in all 5 locations, the locations with lower thermal comfort power consumption rates have higher self-sustainability. According to the data on power supply rates, total energy consumption is always less than yearly power output, and the power supply rate is always greater than 100%. This suggests that all cases can achieve autonomous functioning, with Lhasa at 218.13% and Xining at 106.87%. Lhasa is at the top, followed by Nanning, Hangzhou, Tianjin, and Xining. The locations with higher electricity supply rates are more self-sustainable, and excess electricity output can be used to power other electricity-consuming equipment. It is worth noting that the performance of the appliances used in this simulation was consistent throughout all operating circumstances and did not fluctuate. This study was unable to accurately assess the impact of appliance energy-saving performance on simulation findings.

Table 3 Comparison of performance in 5 locations.
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Technical thermal performance analysis

Because the fecal treatment technology in the autonomous technology of independent public toilets requires some thermal energy, this study estimates the model’s heat generation performance. The thermal efficiency value is taken as 49.83% of the Back Propagation (BP) neural network prediction result of the previous study49. The BP neural network is a widely-used neural network algorithm. Its basic principle is that it consists of an input layer, hidden layers, and an output layer. During the forward-propagation process, the input signals are transmitted through the hidden layers and finally reach the output layer after weighted summation and activation function processing. When there is a deviation between the output and the actual value, the error is calculated and then propagated backward from the output layer to the hidden layers and the input layer. By adjusting the weights of neurons in this process, the network aims to minimize the error between the predicted and actual values, thus achieving better prediction performance. S. Fong and K. W. S. Chong50 applied the group method of data handling neural network approach combined with the BP neural network. For our research, this reminds us to carefully process the data related to independent public toilets before using the BP neural network. For example, we need to select and pre-process relevant environmental and equipment-operation data to better adapt to the BP neural network model.

In our study, when calculating the thermal performance demand, the input parameters of the BP neural network are determined based on various factors. Environmental variables such as ambient temperature (\({T}_{a}\)), solar irradiance, and equipment-operation parameters like the mass flow rate value (\(\dot{m}\)) of the thermal fluid are selected as input parameters. These parameters are crucial as they directly affect the heat generation and consumption in the independent public toilet system. Regarding the hidden-layer settings, after multiple tests and according to the complexity of the problem, we set the number of hidden layers to 2. The number of neurons in the first hidden layer is set to 10, and in the second hidden layer to 8. The basis for this setting is to balance the model’s complexity and computational efficiency. A relatively larger number of neurons in the first hidden layer can capture more complex relationships in the input data, while the second hidden layer further refines these relationships. The output result of the BP neural network represents the predicted thermal energy requirement (\({Q}_{TH}\)). This value is used to analyze the heat-generation performance of the energy-supply technology in the independent public toilet and is also used in subsequent calculations related to heat balance and auxiliary heat-energy demand.

During the training process of the BP neural network, to ensure the model’s generalization ability, we divide the collected data into a training set (70%) and a test set (30%). The training set is used to train the network, allowing the BP neural network to learn the relationships between input and output data. The test set is used to evaluate the performance of the trained model on unseen data. We use the Mean Squared Error (MSE) as the loss function, which measures the average of the squares of the errors between the predicted and actual values. To update the weights, we adopt an improved algorithm. Specifically, we use the Adam optimization algorithm, which combines the advantages of AdaGrad and RMSProp algorithms. It can adaptively adjust the learning rate for each parameter during the training process, accelerating the convergence speed of the model while avoiding over-shooting, thereby improving the training efficiency and the performance of the BP neural network in predicting the thermal performance of independent public toilets. The specific formula for the thermal performance demand of the energy supply technology is Eqs. (1, 2).

$${Q}_{TH}=\sum_{i=1}^{n}\dot{m}\times {C}_{p}\times (25-{T}_{a}) \times 3600$$
(1)
$${Q}_{TH\text{-}total}={Q}_{TH\text{-}day}+{Q}_{TH\text{-}a}$$
(2)

where, \({Q}_{TH}\)—Thermal energy requirement (MJ), where \({Q}_{TH\text{-}total}\) is the thermal energy requirement of the toilet for the whole year of operation, \({Q}_{TH\text{-}day}\) is the thermal energy requirement of the toilet for the whole year of operation with irradiation, and \({Q}_{TH\text{-}a}\) is the thermal energy requirement of the toilet for the whole year of operation without irradiation; \(\dot{m}\) ̇—mass flow rate value, calculated as 0.02 kg/s; \({C}_{p}\)—constant pressure specific heat capacity of thermal fluid, calculated as 1000 J/(kg·°C); \({T}_{a}\)—ambient temperature (°C), when the ambient temperature is greater than 25 °C, calculated as 25 °C; \(n\)—When calculating \({Q}_{TH\text{-}total}\), it is the number of hours of operation of the toilet for the whole year; when calculating \({Q}_{TH\text{-}day}\), it is the number of hours of operation of the toilet with irradiation for the whole year; when calculating \({Q}_{TH\text{-}a}\), it is the number of hours of operation of the toilet without irradiation for the whole year.

The computed month-by-month heat output of the energy supply technologies, as shown in Fig. 4, reveals that the highest month of heat production in all locations (excluding Hangzhou) is May, when spring and summer coincide. The city with the most radiation, Lhasa, produces the most heat month after month, it even approaches 1800 MJ in May. Hangzhou and Nanning have a reputation for producing considerably greater heat in the summer and fall than in the spring. This differs from Tianjin and Xining in the northern region, which could be attributed to the wet season in southern locations, which impacts sun irradiation. Monthly heat output in winter is low in all locations, which is due to the relatively poor solar irradiation in much of China. Hangzhou’s heat generation during the winter months of January and February did not reach 550 MJ. The comparison of the system’s demand for auxiliary heat energy (Fig. 5) shows that the degree of demand is strongly related to the seasonal temperature change. The demand for supplementary thermal energy is exceptionally low in Tianjin from June to August, as well as in Nanning and Hangzhou from May to September, with both being less than 60 MJ and in some months being less than 1 MJ. Except for Nanning, other locations have higher supplemental heat energy consumption during the winter, with an average monthly need of more than 250 MJ. Because of the high altitude, the temperature difference between day and nighttime in Lhasa is large, so the auxiliary heat energy demand is at least 100 MJ month by month. The auxiliary heat energy demand in Xining, which has the highest auxiliary heat energy demand in January, is nearly 450 MJ, which is more than three times that of Nanning.

Fig. 4
figure 4

Comparison of heat production by EAT.

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Fig. 5
figure 5

Comparison of auxiliary heat demand for EAT.

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Equations (1) and (2) may be used to compute heat demand and contribution based on the simulated total heat production. Table 4 shows a comparison of the five locations’ average yearly solar thermal performance indicators. The total heat production from the five locations is sufficient to meet the heat demand of the energy supply technology during the day, which means \({Q}_{th}>{Q}_{TH\text{-}day}\). The effective daytime heat gains \({Q}_{TH\text{-}day}\) at the five locations are Xining, Lhasa, Tianjin, Hangzhou, and Nanning in descending order.

Table 4 Comparison of annual average thermal performance for 5 cases of energy supply technology operation.
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Nanning is the only city among the 5 locations in which the share of solar energy in thermal energy demand has not surpassed 60%. Its low annual thermal energy consumption of 2,558.54 MJ and the high daytime temperatures mean that it doesn’t require excessive thermal energy supplementation. These factors are the major causes of this. With thermal energy contributions of over 60% in the other 4 locations, it is clear that solar thermal utilization may efficiently supply thermal energy for the composting treatment technique of autonomous toilets, hence lowering the need for fossil fuels. It is important to point out though that all 5 locations have large auxiliary thermal energy requirements. While the actual operation of the manure treatment system in the toilets generates its heat within the material, the compost decomposition process can generate temperatures as high as 40 °C51. This part of the heat energy is not calculated, so the system fan may not require additional heat supplementation even after the system fan stops running at night. This calculation is primarily based on the provision of hot air for the composting drums at 25 °C and above.

Analysis of emission reductions

Reducing emissions is of utmost importance in addressing the current global environmental challenges. As pointed out by L. Zeng et al.52 reducing greenhouse gas and other pollutant emissions is a crucial measure to combat climate change and floods, which pose threats to human survival. In the independent public toilet model of this study, the Energy-Autonomous Technology (EAT) reduces the emissions of pollutants such as carbon dioxide, sulfur dioxide, and dust by decreasing the consumption of fossil energy. This not only aligns with the global trend of combating climate change but also contributes to alleviating local environmental problems. For example, reducing emissions helps to slow down the rate of global warming, reducing the probability of sea-level rise and extreme climate events. Reducing sulfur dioxide and dust emissions can improve air quality, protect human health, and reduce health problems such as respiratory diseases. Especially in densely populated urban areas, public toilets, as common infrastructure, the cumulative effect of their emission reduction is of great significance for improving the regional environmental quality.

The EAT for independent public toilets reduces fossil energy consumption and therefore reduces emissions of CO2 (Carbon dioxide), SO2 (Sulphur dioxide), and dust. The CO2 emission reduction \({Q}_{d{co}_{2}}\) of the EAT of independent public toilets can be calculated according to Eq. (3). The SO2 emission reduction \({Q}_{d{so}_{2}}\) can be calculated according to Eq. (4). The dust emission reduction \({Q}_{dfc}\) can be calculated according to Eq. (5). Solar photovoltaic utilization system conventional energy substitution \({Q}_{tr}\) and solar photovoltaic utilization system conventional energy substitution \({Q}_{td}\) are calculated according to Eqs. (6) and (7).

$${Q}_{pvt\text{-}{co}_{2}}= ({Q}_{tr}+{Q}_{td}) \times {V}_{{co}_{2}}$$
(3)
$${Q}_{pvt\text{-}{so}_{2}}= ({Q}_{tr}+{Q}_{td}) \times {V}_{{so}_{2}}$$
(4)
$${Q}_{pvt\text{-}fc}= ({Q}_{tr}+{Q}_{td}) \times {V}_{fc}$$
(5)
$${Q}_{tr}=\frac{{Q}_{nj}}{q\times {\eta }_{t}}$$
(6)
$${Q}_{td}=D\cdot {E}_{n}$$
(7)

where, \({Q}_{pvt\text{-}{co}_{2}}\)—CO2 emission reduction from EAT for independent public toilets, kg; \({Q}_{tr}\)—Conventional energy substitution for solar thermal system, kgce; \({Q}_{td}\)—Conventional energy substitution of solar photovoltaic system, kgce; \({V}_{{co}_{2}}\)—CO2 emission factor of standard coal, calculated as 2.47 kg/kgce. \({Q}_{pvt\text{-}{so}_{2}}\)—SO2 emission reduction of EAT for an independent public toilet, kg; \({V}_{{so}_{2}}\)—SO2 emission factor of standard coal, calculated as 0.02 kg/kgce. \({Q}_{dfc}\)—Dust emission reduction of EAT for an independent public toilet, kg; \({V}_{fc}\)—Dust emission factor of standard coal, calculated as 0.01 kg/kgce. \({Q}_{nj}\)—Effective heat gain of solar thermal utilization system for the whole year, MJ; \({\eta}_{t}\)—Operating efficiency when using conventional energy as the heat source, taken as 0.70 in this study; \(D\)—The amount of standard coal consumed per kWh of electricity converted, calculated as 0.123 kgce/kWh (according to the conservation of energy, 3.6 MJ is equivalent to 1 kWh of electricity, divided 3.6 MJ by 29.307 MJ/kgce); \({E}_{n}\)—Annual power generation of the solar PV system, kWh.

By plugging the simulation data into Eq. (3) to (7), the corresponding coal savings and emissions reductions may be determined. Incorporating the traditional energy replacement produced by photovoltaic and solar thermal utilization technologies for each of the 5 cases into the computation, Table 5 displays the emission reductions for each of the three indicators. Lhasa has the largest emission decrease of the three indicators, with Xining, Tianjin, Hangzhou, and Nanning following in order of decreasing emissions. Out of the 5 locations, the locations with higher ambient temperatures have smaller emission reductions of the three metrics and save less coal. This is mostly because there is less of a need for light and heat in these locations than there is in colder climates, where there is a larger need for both, leading to a bigger reduction in emissions.

Table 5 Comparison of coal savings and emissions reductions in 5 locations.
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Characterization of climate adaptation

The 3 locations with low winter temperatures, Xining, Tianjin, and Lhasa, all have a large need for thermal energy from the output of the EAT, as independent public restrooms have certain operational temperature requirements. In the locations with cold winters, all of the design alternatives have higher energy usage. Even if more cooling energy consumption is added to suit summer comfort needs, it will not affect the ordering of energy consumption per square meter since summertime high ambient temperatures are good for excreta disposal. This is because additional cooling energy consumption is modest35,40. Given the thermal energy demand of toilets, the locations with higher ambient temperatures are more conducive to the application of EAT. These findings are based on the two data sets of thermal comfort power consumption rate and power supply rate. Additionally, irradiance and ambient temperature are critical for the operation of EAT.

There is less of an emission decrease in Hangzhou and Nanning than in Lhasa, Xining, and Tianjin because of their higher ambient temperatures, which also reduce their need for thermal energy and the energy consumption of the entire model. On the other hand, the three locations of Tianjin, Xining, and Lhasa enjoy favorable solar radiation conditions. Additionally, the model itself can leverage technology to generate more electrical and thermal energy, allowing for the application of EAT in public toilets and optimizing benefits under identical input conditions. In the locations where there is less of a need for heat energy, the PV/T can use electric energy in place of collecting heat energy, which can lower costs and somewhat increase efficiency35.

Discussion

In the event of extreme climatic conditions, such as persistently low solar irradiation, where the amount of electricity and heat produced by PV/T technology is insufficient to meet daily demands, storage batteries, and auxiliary heating equipment must be installed in the system to maintain energy supply-demand balance. Based on the energy-use characteristics of different regions, independent public toilet technology may require EAT to regulate energy during the winter and summer, which are two seasons of high energy consumption respectively. The highest average daily energy use in the five locations for the entire year is approximately 3.25 kWh. With the current battery type typically used for PV equipment (12 V, 250 AH, 3 kVA), four batteries connected in series can meet the electrical needs of the toilets for three days of operation in the Xining area under extreme weather. The use of solar energy for energy supply in the five locations indicated above may entirely fulfill utilization needs while also significantly reducing greenhouse gas emissions. In other locations where solar energy is insufficient, wind energy can be used as a complement53.

When comparing the energy consumption data of public toilets in other similar studies, it is found that most traditional public toilets rely heavily on external municipal energy supplies, such as grid electricity and central heating. Their energy consumption is high, and they are significantly affected by the stability of external energy supplies. In contrast, the independent public toilets with the EAT in this study show significant differences in energy consumption in different regions, but generally present a more energy-efficient trend. Taking Tianjin as an example, the energy consumption of the independent public toilet in this study is 207.99 kWh/m², which is lower than the energy consumption data of some traditional public toilets.

The advantages of EAT in this study lie in its utilization of renewable energy sources such as solar energy, reducing dependence on external energy, effectively lowering energy costs and carbon emissions. Through the Photovoltaic/Thermal (PV/T) technology, it realizes the photoelectric and photothermal conversion of solar energy, providing energy for the operation of equipment and excreta treatment in public toilets. In areas with sufficient sunlight, a high energy self-sufficiency rate can be achieved. For instance, in Lhasa, where solar radiation is strong, the power supply rate of the EAT system is as high as 218.13%, which not only meets its own needs but also has surplus electricity for other devices.

However, EAT also has some disadvantages. In some areas with insufficient sunlight or extreme climates, such as high-latitude regions with long and cold winters, the acquisition of solar energy is limited, and the energy supply may not fully meet the demand. It is necessary to rely on auxiliary energy equipment, which increases the system cost and complexity to a certain extent. In addition, compared with some public toilets that efficiently use municipal energy and are equipped with advanced energy-saving devices, the initial investment and maintenance costs of the EAT system are relatively high, which may limit its popularization and application in some areas with limited budgets.

Under different climatic conditions, the thermal performance of the EAT system varies significantly, which has a crucial impact on its practical applications. In regions with high solar radiation and relatively high temperatures, such as Nanning, the heat production of the EAT system is sufficient during most of the year. The high-temperature environment is conducive to the composting process in the excreta treatment system, reducing the demand for auxiliary heat energy. This means that the system can operate more stably and efficiently, with lower energy costs. However, in areas with cold winters and relatively low solar radiation, like Xining, the heat production of the EAT system is insufficient in winter. The low-temperature environment requires a large amount of thermal energy to maintain the normal operation of the excreta treatment system and indoor comfort temperature, which increases the demand for auxiliary heat energy. This not only raises energy costs but also challenges the stability of the energy supply system.

To optimize the energy supply technology according to these results, in areas with sufficient sunlight, we can further expand the scale of PV/T devices to increase the capture and utilization of solar energy. For example, we can increase the area of PV panels around the toilet module or adjust the angle of the roof PV panels according to the seasonal changes of sunlight to improve the efficiency of heat collection. In cold regions, we can consider integrating other renewable energy sources, such as wind energy, with solar energy. When solar energy is insufficient in winter, wind energy can be used as a supplement to ensure the stable operation of the energy supply system. Additionally, improving the heat-storage capacity of the system, such as using high-performance heat-storage materials, can help store excess heat energy generated during the day for use at night or on cloudy days, enhancing the adaptability of the EAT system to different climatic conditions.

It works best in places with low temperatures and high sun irradiation if the external power output is ignored. Among the five locations, Xining has the lowest power supply rate, suggesting the greatest advantage from technological application; yet, since it cannot generate cooling energy during the summer, it has a low rate of electric energy utilization during that season. The greater the city’s irradiance, in terms of several emission reduction metrics, the greater the effect of coal saving and emission reduction. Because Tianjin consumes more energy in the summer and less in the winter and maintains a more balanced energy use throughout the year, EAT performs better in climate zones with strong solar radiation and low ambient temperatures, such as the one in which Tianjin is situated.

Conclusion

The EAT for independent public restrooms can operate independently in all five locations, according to a climate resilience study of the model. The following are the study’s primary conclusions. The findings suggest that Nanning is the best option in places with high light intensity and high temperatures, with an average maximum daily energy usage of around 3.25 kWh, but Xining has a higher benefit in areas with high light intensity but lower temperatures. In terms of electric heat supply, Tianjin generates 13,062.58 MJ of heat each year, with an effective heat gain of 3,089.97 MJ and an auxiliary heat demand of 5,050.08 MJ, resulting in a 61.19% heat contribution rate. Battery management systems may be required in regions such as Lhasa, Xining, and Tianjin to maintain the balance of energy supply and demand. Lhasa achieved 887.49 kg/a, 7.19 kg/a, and 3.59 t/a in CO2 reduction, SO2 reduction, and dust reduction, respectively, demonstrating the significant benefits of EAT at high altitudes. Locations like Xining, Tianjin, and Lhasa that experience cold winters require more thermal energy from autonomous technology, and the energy ranking is unaffected by summer increases in cooling energy consumption. Higher irradiance locations have greater effects on reducing emissions, and Tianjin’s more balanced summer and winter energy consumption makes it an excellent location for autonomous technological applications. The long-term growth of independent public toilets, energy efficiency, environmental protection, infrastructural development, and practical applications are all greatly aided by these discoveries. To provide clean and hygienic toileting conditions for residents, a more sustainable, effective, and environmentally friendly public toilet system can be built through intensive research and the promotion of EAT technology in independent toilets. This is especially true for areas with limited water resources or inadequate infrastructure.

Solar energy is used as an energy substitute by the EAT for independent public toilets, which do not depend on an outside municipal energy source for operation. By producing thermal energy and electrical power for consumption during toilet operations and fecal disposal, solar energy successfully addresses the issue of high environmental loads. Independent public toilets with EAT can be arranged according to the needs of use without being constrained by water and energy sources, offering a new concept for the promotion and application of toilets in water-scarce or impoverished areas where clean, sanitary toilets are not widely available.