Abstract
The reason for utilizing PV-T systems is their capability to generate both electricity and heat at the same time, which enhances energy efficiency and decreases the need for multiple energy sources. Considering the potential of solar energy and different climates in Iran, in this research, the feasibility, modeling, and comparison of the PV-T system to supply electricity and heat to residential units were investigated in 28 cities with 9 climates of Iran. A thorough evaluation of performance was conducted using the software Polysun 9.0 and PVsyst 6.6.8 for a duration of one year. The scientific novelty of this study relies on the simultaneous utilization of two analytical software, the thoroughness of the research, and taking into account the impact of climate on the findings. Investigations showed that the best performance of the designed system is related to Bandar Abbas and Zahedan, and the worst performance is related to Rasht and Sari. In general, according to the amount of electricity and heating produced by the PV-T system, it showed that Shiraz and Rasht stations have the highest and lowest production of energy. In terms of environmental effects, Birjand station will have the most and Rasht the least reduction in CO2 emissions. In terms of heating energy demand for the investigated building, Ardabil has the highest amount and Bandar Abbas has the lowest amount.
1. Introduction
Energy supply technology is currently facing a range of challenges, including price fluctuations, concerns about energy security, and environmental issues [1]. Environmental degradation, depletion of natural resources, global warming, air pollution, climate change, widespread droughts, and other similar threats have compelled many countries to reduce their reliance on fossil fuels and transition to cleaner energy sources [2, 3]. Given these problems, it appears that developing renewable energy in accordance with the potential of each region while also making rational use of fossil fuels is a suitable solution [4, 5]. These issues serve as the primary drivers for the creation of new and more efficient electricity production systems [6]. Therefore, it is undeniable that investigating the potential and feasibility of various technologies related to renewable energy is of utmost importance [7].
In Iran in 2020, fossil fuel-based electricity accounted for over 86% of the country’s electricity supply portfolio. Iran holds the top position globally in terms of underutilizing its capacity [4]. Moreover, Iran ranks among the top ten carbon producers worldwide and stands as the largest producer of CO2 in the Middle East. Solar energy emerges as a promising technology that can contribute to transforming Iran into a sustainable society [8].
Renewable energies currently provide approximately 15% of the final energy demand in buildings. However, there continues to be an increase in energy demand within buildings. From 2009 to 2019, there was a yearly increase of 5.3% in renewable electricity used for heat generation in buildings. In contrast, the portion of electricity used for building heating only increased by 1.3% during this period [9]. Energy consumption in buildings has also experienced a gradual increase from 2009 to 2019, with an average annual growth rate of 1%. However, the COVID-19 pandemic had a temporary impact on this trend, leading to a slight decrease in energy requirements as many public and commercial buildings transitioned to low-energy operations. Preliminary estimates suggest that as economic activities resume in 2021, building energy consumption will return to its previous high levels. It is worth noting that approximately one-third of the world’s total energy consumption is directly used in buildings, with thermal energy accounting for about 77% and electrical energy for the remaining 23% [9]. Therefore, focusing on this sector can significantly contribute to reducing nonrenewable energy consumption.
One effective method for harnessing solar energy is through the use of PV-T collectors [10, 11]. These devices have the advantage of producing a higher energy output per unit area compared to solar thermal collectors or photovoltaic panels [12]. Due to the growing interest in electrified heating systems, there is expected to be an increased demand for PV-T collectors in 2021. In fact, thirty manufacturers reported sales of at least 88 MWh of PV-T systems within the year, representing a significant growth of 45% compared to the 61 MWh sold in 2020. The largest emerging markets in this industry include France, the Netherlands, Israel, Germany, and Spain [9]. Researchers believe that the PV-T market is still in its early stages. As of the end of 2020, there were approximately 1,275,431 m2 of PV-T systems operating worldwide. Moreover, an average annual market growth rate of around 9% has been observed from 2017 to 2020 [13].
Up until now, there have been numerous extensive studies conducted on the different types, components, working fluids, and optimization methods of the PV-T system. Overall, scholars have commonly utilized manual and field-laboratory methods, as well as various software tools (as shown in Table 1).
Based on the research literature, there have been few studies that have focused on the potential efficiency of PV-T in different climates. Some researchers, like Farahani and Alibeigi [29], have examined PV-T in Iran. They investigated the performance of photovoltaic-thermal-thermoelectric generators in several Iranian cities. Their findings showed that the performance is influenced by temperature and radiation intensity. Additionally, the amount of electricity generated increases with more thermoelectric generator modules. Kerman City had the highest thermoelectric generator efficiency, while Bojnord City had the highest PV-T efficiency. In another article, Farahani et al. studied a combined-cycle PV-T heat exchanger to provide air conditioning for a 200 m2 building in nine Iranian cities using Carrier software. They found that the power utility of the PV-T system increased by 5% in winter and 8% in summer, with Tabriz City having the highest power efficiency. Kerman had the highest electrical energy usage [30]. However, specific emission reduction statistics and climate data for all cities were not provided in their research. Given the issues associated with nonrenewable energy consumption and Iran’s position in fuel consumption and pollution production, as well as its abundant solar radiation potential and significant use of housing for thermal and electrical purposes, this study is aimed at evaluating the technical, economic, and environmental aspects of implementing residential PV-T systems in Iran. The results will be compared to previous research findings.
This study provides a comprehensive analysis of the feasibility, modeling, and comparison of PV-T systems to supply electricity and heat to residential units in 28 cities with 9 different climates in Iran based on the Köppen climate classification. The importance of the study lies in the assessment of the feasibility and performance of PV-T systems for residential use in Iran, considering the country’s diverse climates and the urgent need to shift towards renewable energy sources to reduce environmental pollution and dependence on fossil fuels. The findings of this study provide valuable insights into the environmental and economic benefits of using PV-T systems in residential units, which can guide policymakers, researchers, and engineers in the implementation of renewable energy solutions in Iran and other regions with similar climate conditions.
2. Methodology
The research methodology used in this study is a quantitative approach. The study uses modeling and comparison of the PV-T system to supply electricity and heat to residential units in 28 cities with 9 climates of Iran. Based on the weather database in the Meteonorm 7.3.3 software, an effort has been made to choose several stations in different regions of Iran. This approach ensures that the findings of this study are comprehensive and the results obtained are more trustworthy. Performance analysis has been done using Polysun 9.0 and PVsyst software. The study also includes an environmental assessment by examining the reduction in CO2 emissions. In this study, the initial step involved determining the optimal angle for each city using PVsyst 6.6.8 software. Subsequently, the system and demand information were inputted into Polysun 9.0 for simulation purposes. The advantages and disadvantages of electricity production, heat, total power output, cost of power production, losses, and emissions were evaluated for different cities in Iran based on specific goals. The simulation plan for the system and its components in Polysun 9.0 was taken into account to achieve these objectives.
Figure 1 illustrates the structure of a PV-T system. The overall efficiency () of the PV-T system is influenced by factors such as air mass flow rate, solar collector type, absorber variations (e.g., baffles and thin metallic sheets in the cooling duct), sheet and tube absorber, roll band absorber, and temperature [25] (Table 2). Table 2 also presents calculations for system performance, CO2 reduction, and fuel savings.

3. Case Study Location
Iran is situated in the southwestern part of Asia, specifically in the Middle East region, which is known for its vast deserts and clear skies. With a total area of 1,648,195 km2, Iran has ample space for renewable energy sources. Each region within the country possesses different potential for solar energy [8]. Iran benefits from a diverse climate, which increases the feasibility of utilizing renewable energy sources [4].
The Köppen classification system, widely used by researchers, has undergone modifications over time due to criticisms [32]. The updated Köppen-Geiger classification system is now commonly used [33]. Recent surveys conducted between 1990 and 2014 reveal that out of the 31 climate groups identified by Köppen-Geiger, Iran encompasses nine of them (see Figure 2) [34].

(a)

(b)
Electricity usage in residential, industrial, agricultural, and commercial sectors is increasing [36]. Gas and oil account for 99% of energy production in Iran, while renewable energies only make up 1%. It is worth noting that Iran is situated in a region with high solar radiation, with an annual average of about 20 to 30 MJ/m2. In central regions, this value is even higher [37] (Figure 3).

Iran has abundant sources of renewable energy, particularly solar energy, due to its high solar radiation and vast available land. This suggests that the use of solar systems in Iran would be cost-effective. However, despite this potential, Iran’s solar market remains undeveloped [8]. Currently, there are 103 MW of renewable power plants already installed in the country, with an additional 42 MW under construction. In terms of the country’s renewable power plants, 44% are solar, 40% are wind, 13% are small hydropower, 2% are heat recovery, and 1% are biomass [4].
4. Input Data
Based on Figure 4, the standards state that an average family of four consumes approximately 10 kWh of electricity per day, with an annual consumption of around 3500 kWh. Additionally, the daily hot water demand is 200 L. The proposed model consists of a residential unit spanning 148 m2 across two floors, equipped with a 7.5 kW gas boiler, an 800 L tank, a 2.5 kW PV-T panel, and a pump. The specifications for each component are inputted into the software. The feasibility and economic-environmental aspects of implementing the PV-T system in various cities and regions in Iran (28 cities and 9 regions) have been assessed using Polysun 9.0 simulation software.

5. Results
Table 3 provides the average annual outdoor temperature in °C and the amount of radiation on the surface of the collectors in kWh for the stations being studied. This includes the effective energy, taking into account losses from the incidence angle modifier (IAM) and shadows, also measured in kWh. The station with the highest annual radiation on the collectors’ surface is Shiraz, with 38,388 kWh, while Rasht has the lowest at 23,594 kWh. Similarly, Shiraz and Rasht have the highest and lowest annual effective energy values, respectively, considering IAM losses and shadows.
The optimal angle values for installation were determined using PVsyst 6.6.8 software. The calculations were based on comparing the drop percentage to zero to achieve an optimal state. The software provided an angle limit, and the maximum angle listed in the table for each station was considered the installation angle. According to simulation results, Bojnoord has the highest annual optimal angle at 39 degrees, while Bandar Abbas has the lowest at 27 degrees. Table 3 also presents data on annual electricity generation from photovoltaic panels integrated with thermal collectors (PV-T collectors) in terms of direct current (DC) and alternating current (AC) electricity production measured in kWh. Shiraz has the highest electricity production at 4,315 kWh, while Rasht has the lowest at 2,741 kWh (Figure 5). Analyzing the annual energy provided by the collectors in terms of heat transfer to working fluid (excluding losses from pipes and tanks), Birjand has the highest value at 3,661.3 kWh, whereas Rasht has the lowest value at 1,875.7 kWh (Table 3 and Figure 5).

The system performance parameter allows for an objective comparison between different systems. A higher value indicates better results, which is obtained from Equation 5 [39]. In this case, the system designed in Bandar Abbas and Zahedan has the best performance with values of 4.67 and 1.9, respectively, while the worst performance is seen in the system in Rasht and Sari with values of 0.88 and 0.96, respectively.
These results demonstrate the ability of the designed system to meet the energy needs of different stations. However, it is evident that the designed system is unable to supply the required energy for Arak and Hamadan stations. Therefore, an upgrade is necessary for these stations. The heating energy demand for the designed building is equivalent to the annual amount of energy provided by the radiators in the heated space. Additionally, simulations and analyses reveal that Bandar Abbas has supplied the highest percentage of energy needs through solar energy at 91.6%, while Ardabil has supplied the lowest amount at 22.6%. Similarly, when considering hot water supply through solar energy, Bandar Abbas has provided the highest percentage at 91.6%, while Rasht has provided the lowest amount at 39.8%. Furthermore, simulations show that Bandar Abbas has supplied the highest percentage of average energy required for heating a house through solar energy at 91.6%, whereas Ardabil has supplied only 8.8%. To calculate CO2 emission prevention, we multiply the amount of solar energy in the tank () by the CO2 emission from fuel and divide it by the annual efficiency of the boiler (Equation 6).
The column regarding the decrease in CO2 emissions indicates that Birjand station will have the greatest reduction in CO2 emissions, with a value of 942.1 kg, while Rasht station will have the lowest reduction, with a value of 482.7 kg (Table 4 and Figure 6).

The software has calculated the solar energy received from the windows in kWh and the total annual energy loss through the building and air exchange. Urmia station has the highest amount of total annual energy losses through building and air exchange, at a rate of 38745.8 kWh, while Ahvaz station has the lowest amount at a rate of 22530.6 kWh. The following table displays the annual heat loss through the tank wall and connections (Table 5). The highest and lowest costs of producing AC electricity and electrical-thermal energy are associated with Rasht stations, at amounts of 0.511 and 0.530 $/kWh, respectively, while Shiraz has amounts of 0.324 and 0.308 $/kWh, respectively. Table 5 shows the maximum savings in annual fuel consumption achieved through the use of solar energy technology, measured in m3 of natural gas equivalent and kWh equivalent. The highest savings are seen in Birjand station at 387.4 m3, while Rasht station has the lowest savings at 198.5 m3 (Table 5 and Figure 7). According to Equation 7, which calculates energy savings, the solar energy in the tank () is divided by the calorific value of the fuel and the total annual efficiency of the boiler.

6. Conclusion
This research has demonstrated the feasibility and potential benefits of utilizing PV-T systems in residential units in Iran. The ability of these systems to generate both electricity and heat simultaneously enhances energy efficiency and reduces the reliance on multiple energy sources. By investigating 28 cities with 9 different climates in Iran, this study has provided a comprehensive evaluation of the performance of PV-T systems. The use of analytical software such as Polysun 9.0 and PVsyst 6.6.8 has allowed for accurate modeling and comparison of the system’s capabilities over a one-year duration. The scientific novelty of this research lies in the simultaneous utilization of two analytical software, which enhances the reliability and accuracy of the findings. Additionally, the thoroughness of the study in considering the impact of climate on the performance of PV-T systems adds further value to its conclusions. The results obtained from this research highlight the potential for widespread adoption of PV-T systems in Iran, given its abundant solar energy resources and diverse climates. Implementing these systems can contribute to reducing greenhouse gas emissions, improving energy efficiency, and decreasing dependence on traditional energy sources. Overall, this study provides valuable insights into the feasibility and benefits of utilizing PV-T systems for residential units in Iran. It serves as a foundation for further research and encourages policymakers to consider implementing these systems as part of their sustainable energy strategies. The investigation found that: (i)Bandar Abbas and Bojnoord have the lowest and highest annual optimum angles, respectively(ii)Shiraz has the highest radiation on the surface of collectors, while Rasht has the lowest(iii)Bandar Abbas and Zahedan have the best system performance, while Rasht and Sari have the worst(iv)Shiraz has the highest AC electricity generation, while Rasht has the lowest(v)Ardabil has the highest heating energy demand, while Bandar Abbas has the lowest(vi)Bandar Abbas has the highest percentage of energy supplied through solar energy, while Ardabil has the lowest(vii)Birjand has the highest reduction in CO2 emissions, while Rasht has the least(viii)Urmia has the highest total energy loss, while Ahvaz has the lowest(ix)Rasht has the highest cost of electricity production, while Shiraz has the lowest(x)Birjand has the highest savings in fuel consumption, while Rasht has the lowest
7. Suggestions for Further Work
For future research, it is suggested that these scenarios be investigated for other applications such as administrative, commercial, educational, industrial, and medical spaces. Also, the method used in this research can be applied to other countries with different potentials. Another topic that can be of interest to researchers is calculating the amount of exergy in the investigated systems. Also, by installing solar systems on different sites, the experimental results can be compared with the results of the present work.
Nomenclature
: | Heat generator fuel and electricity consumption (kW) |
: | AC power generation (kW) |
: | Electricity consumption of pumps (kW) |
L/day: | Liters per day |
MWp: | Medieval warm period |
: | Energy supplied by collectors (kW) |
: | Total energy consumption (kW) |
: | The solar energy in the tank (kW) |
: | Total energy efficiency of PV-T systems (-) |
: | Thermal efficiency (-) |
: | Electrical efficiency (-) |
: | Collector area (m2) |
: | Coolant specific heat capacity (J/(kg·°C)) |
: | Electric current (A) |
: | Collision of solar irradiation (W/m2) |
, : | Temperatures of coolant (air/water) in inlet and outlet, respectively (°C) |
: | Module temperature (°C) |
: | Reference temperature (°C) |
: | Maximum power point operation voltage (V) |
: | Working fluid mass flow rate (kg) |
: | Temperature coefficient (-) |
: | Reference yield of PV module (-) |
PV-T: | Photovoltaic-thermal (-) |
IAM: | Incidence angle modifier (-) |
DC: | Direct current (-) |
AC: | Alternating current (-). |
Data Availability
All data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.