*1.1. Background and Existing Studies*

The concept of "electrify everything" considers solar energy as a key renewable technology with an aim of de-carbonization of domestic heating demand [1]. The rapid growth in photovoltaic (PV) installation capacity from the last few years has further strengthened the importance of PV as the main driver of renewable transformation [2]. PV remains an interesting subject area for many researchers, global leaders, and manufacturers because of its reliability, sustainability, ease of installation, and economic feasibility [3]. However, the concurrence of heat/electricity demand and limited roof area in domestic dwellings does require technologies which can generate energy efficiently in both thermal and electrical form. Therefore, there is a huge potential for well-designed systems by combining both solar PV and solar thermal technologies. A relatively new commercialized concept of solar photovoltaic/thermal (PVT) technology can achieve such a goal by generating both electrical and thermal energy together using a single panel [4]. Realizing its importance, the Solar Heating and Cooling Program (SHC) of the International Energy Agency (IEA) has initiated Task 60 for PVT applications and solutions to Heating, Ventilation and Air Conditioning (HVAC) systems in buildings [5]. The task has been active from January 2018 and has built a huge knowledge base around PVT systems for its use in domestic and industrial applications.

PVT systems can be categorized in several ways, however, the most common is based on the heat-transfer medium (air-based/liquid-based) used in the PVT collector [6]. The liquid-based types are dominating the current PVT market in terms of the number of installations due to high efficiency, and ease of integration in existing hydronic systems [7]. In a standard liquid-based PVT collector, the heat carrier is usually water or brine mixture, which is allowed to circulate in a heat exchanger behind the PV cells. The circulation results in a heat transfer through the back sheet of the module, which raises the fluid temperature enough to use for various applications such as, e.g., hot water and swimming pool heating. From a technical perspective, PVT technology is well developed, and it can be coupled with various energy systems. For instance, it can go hand-in-hand with the emerging awareness of heat pump technology with/without borehole storage [8]. However, the current main barriers in PVT development and deployment are lack of testing standards, uncertain financial incentives, and business models across different regions in a niche market. Therefore, the business potential of PVT solution has not been fully explored, although it can be a very efficient solution for domestic and industrial heating requirements.

There are several studies concerning the techno-economic analysis of PVT collectors with a focus on the component and system design [4,9–12]. The most common way is to assess the energetic performance firstly and then carry out an economic evaluation based on dependent variables [4,9,10,12–16]. The prevalent energy performance indexes are energy efficiency and exergy efficiencies [6] while the most popular economic indicators are represented by levelized cost of energy (LCOE), net present value (NPV), and payback period [4]. To name a few studies for technical evaluation, Fudholi et al. [13] investigated electrical and thermal performances on PVT water-based collectors by testing with specific inputs parameters ranging from 500 to 800 W/m<sup>2</sup> solar irradiance and mass flow rate of 0.011 to 0.041 kg/s. The test concluded that absorber performed better at a mass flow rate of 0.041 kg/s and under 800 W/m<sup>2</sup> irradiance, with a measured PV efficiency of 13.8%, thermal efficiency of 54.6%, and overall collector efficiency of 68.4% [13]. Shah and Srinivasa [17] developed a theoretical model using COMSOL multi-physics validation tool with standard test conditions (STC) to measure the PV improved efficiency when it is integrated with hybrid PVT system. Another study performed by Buonomano [18] developed a numerical model to conduct the technical and economic analysis of PVT collectors and compared it with conventional PV collectors installed in Italy. The tool was validated using TRNSYS platform for the energetic and economic performance of systems integrated with PV and PVT collectors together. Yazdanpanahi [19] presented a numerical simulation and experimental validation for evaluation of PVT exergy performance using a one-dimensional steady thermal model and a four-parameter current–voltage model for a PVT water collector. In terms of economic studies, Gu et al. [4] developed an analytical model on basis of combinations of Monte Carlo method to analyze techno-economic performances of solar PVT concentrator for Swedish climates, which considered several essential input uncertainties whereas economic variables were initially assessed. The developed model has expressed results for capital cost range between 4482 and 5378 SEK/m<sup>2</sup> for 10.37 m<sup>2</sup> system cost during the system lifespan of 25 years. The paper results indicated an LCOE of 1.27 SEK/kWh and NPV of 18,812 SEK with a simple payback period of 10 years. It was concluded that the most

important sensitivity factor is average daily solar irradiation followed by debt to equity ratio, capital price, regional heating price, and discount rate. Herrando et al. [20] performed techno-economic analysis of hybrid PVT systems for electricity and domestic hot water (DHW) demand for a typical house in London and concluded that such systems can meet 51% of electricity demand and 36% of DHW demand even during low solar global horizontal irradiation (GHI) and ambient temperatures. In the economic aspect, it was also concluded that hybrid PVT technology has better energy yield per unit roof area, which can result in attractive NPV for investor while mitigating the CO<sup>2</sup> emissions. Riggs et al. [10] developed a combined LCOE techno-economic model for different types of hybrid PVT systems applied for process heat application in the United States. The sensitivity analysis of parameters affecting the levelized cost of heat (LCOH) was determined using technical, financial, and site-specific variables. Ahn et al. [21] studied the importance of energy demands, solar energy resources, and economic performances of hybrid PVT systems at different PV penetration levels using Monte Carlo method, whereas the study found that irrespective of PV penetration levels, the uncertainties in energy demands and solar irradiance can influence the energy performance of PVT systems. Heck et al. [22] conducted Monte Carlo method for LCOE based on probability distribution, which concluded that this method provides more realistic information on risk/uncertainty, which triggers more scope of potential investment on electricity generation. However, author defended that the method is slightly complex to use point values.

There is more literature available regarding PVT techno-economic performance than what is presented in this study. However, most of the existing studies focused on a single climate, with a straightforward economic–financial analysis. Furthermore, complicated procedures or individual software (e.g., TRNSYS, Polysun) are used to estimate the performance of PVT collectors, which require detailed modelling skills, and higher computation time. There is a lack of a comprehensive simulation of PVT techno-economic performance through a common tool over a large geographic area, aiming for application feasibility and business potentials. Moreover, many studies have reported the solar energy resource potential of buildings at different spatial scales using digital mapping methods, such as digital numerical maps [23], digital surface model [24], satellite imageries and geographic information systems [25,26], and multi-scale uncertainty-aware ranking of different urban locations [27], which provide direct evaluations for solar application, leading to robust planning decisions. Nevertheless, no study has yet been found for mapping of techno-economic performance of PVT systems.

As a result, this paper aims to fill this research gap by utilizing a validated simulation tool to perform a comprehensive techno-economic performance simulation for a wide range of cities. The results are further analyzed and visualized using a digital numerical mapping approach to establish a comparison among various regions.

#### *1.2. Aim and Objectives*

This study aims at simulation and mapping of the energetic and economic indicators of a typical PVT system over different regions to establish a digital performance database for various key performance indicators (KPIs). The economic feasibility of the PVT collector is obtained and compared under various financial scenario models. The data obtained from simulations are used to establish a simple correlation between variables affecting the PVT system.

The main objectives of this paper are to:


The significance of this paper lies in (1) understanding of typical PVT components behavior at the system level and (2) mapping of the collector energetic and economic performance for different climatic conditions across the world. This research results would reflect the concrete developments in this subject area and help the promotion of potential markets, e.g., discovering the economic feasibility of the PVT system and feasible financial solutions to the PVT system in different regions. This paper evaluates the related business benefits of a typical PVT system, which would help to develop a database as repository of PVT performances in different regions and contexts. The research results will be useful for researchers, planners, and policymakers to further evaluate PVT potentials in a net-zero/positive-energy district towards energy surplus and climate neutrality. *Buildings* **2020**, *10*, x FOR PEER REVIEW 4 of 30 feasibility of the PVT system and feasible financial solutions to the PVT system in different regions. This paper evaluates the related business benefits of a typical PVT system, which would help to develop a database as repository of PVT performances in different regions and contexts. The research results will be useful for researchers, planners, and policymakers to further evaluate PVT potentials in a net-zero/positive-energy district towards energy surplus and climate neutrality.

#### **2. System Description and Research Methodology 2. System Description and Research Methodology**

#### *2.1. Water-Based PVT Collector 2.1. Water-Based PVT Collector*

Among the different types of PVT technology, the water-based PVT is the most common one that has great possibilities for system integration [28]. This PVT collector type is structured similarly to the typical flat-plate collector, as shown in Figure 1. It is a sandwiched structure comprising several layers, including a glass cover placed on the top, a layer of PV cells or a commercial PV lamination laid beneath the cover with a small air gap in between, heat-exchanging tubes or flowing channels through the absorber and closely adhered to the PV layer, and a thermally insulated layer located right below the flow channels. All layers are fixed into a framed module using adequate clamps and connections. In the heat-exchanging tubes, water is the most commonly used heat carrier medium due to high specific heat capacity and ease of availability. The glass cover is often optional depending on the system design priority for the type of output required (i.e., electricity or heat). The glass cover helps to reduce heat convection losses, but it also causes high solar reflectance losses and thus lowers optical efficiency. In many cases, the glass cover is used when higher heat output is expected, while it is removed when the system is optimized for higher electrical output. Among the different types of PVT technology, the water-based PVT is the most common one that has great possibilities for system integration [28]. This PVT collector type is structured similarly to the typical flat-plate collector, as shown in Figure 1. It is a sandwiched structure comprising several layers, including a glass cover placed on the top, a layer of PV cells or a commercial PV lamination laid beneath the cover with a small air gap in between, heat-exchanging tubes or flowing channels through the absorber and closely adhered to the PV layer, and a thermally insulated layer located right below the flow channels. All layers are fixed into a framed module using adequate clamps and connections. In the heat-exchanging tubes, water is the most commonly used heat carrier medium due to high specific heat capacity and ease of availability. The glass cover is often optional depending on the system design priority for the type of output required (i.e., electricity or heat). The glass cover helps to reduce heat convection losses, but it also causes high solar reflectance losses and thus lowers optical efficiency. In many cases, the glass cover is used when higher heat output is expected, while it is removed when the system is optimized for higher electrical output.

The electrical efficiency of PV cells increases when the pumped cooled water flows across the rigid series or parallel tubes. The flow control is an important factor to achieve overall high performance of the PVT collectors [29]. In addition to electricity production, hot water is generated by absorbing extra heat from the PV layer, which can be used for several applications. The electrical and thermal efficiencies of PVT generally depend on the PV cell type, fluid temperature, fluid flow rate, flow channel size/configuration, and ambient climatic conditions. The collector energetic performance can be measured in terms of energy utilization ratio and exergy efficiency [19]. The electrical efficiency of PV cells increases when the pumped cooled water flows across the rigid series or parallel tubes. The flow control is an important factor to achieve overall high performance of the PVT collectors [29]. In addition to electricity production, hot water is generated by absorbing extra heat from the PV layer, which can be used for several applications. The electrical and thermal efficiencies of PVT generally depend on the PV cell type, fluid temperature, fluid flow rate, flow channel size/configuration, and ambient climatic conditions. The collector energetic performance can be measured in terms of energy utilization ratio and exergy efficiency [19].

**Figure 1.** Schematic cross-section of a covered flat-plate photovoltaic thermal (PVT) collector [30]. **Figure 1.** Schematic cross-section of a covered flat-plate photovoltaic thermal (PVT) collector [30].

This paper will focus on a typical PVT collector developed by a Spanish manufacturer named Abora solar. The collector is available on the market, and more than 5700 m2 of the gross collector is installed for a broad range of applications. The collector is a covered PVT type with an additional layer of glass on the top of the collector (in addition to a glass layer for PV cells) to reduce the heat This paper will focus on a typical PVT collector developed by a Spanish manufacturer named Abora solar. The collector is available on the market, and more than 5700 m<sup>2</sup> of the gross collector is installed for a broad range of applications. The collector is a covered PVT type with an additional layer of glass on the top of the collector (in addition to a glass layer for PV cells) to reduce the heat convection

convection losses. The rated power of the collector is 365 W at standard test conditions (STC) with a

losses. The rated power of the collector is 365 W at standard test conditions (STC) with a collector area of 1.96 m<sup>2</sup> consisting of 72 monocrystalline cells. The main specifications and characteristics of analyzed PVT collector are shown in Table 1.


**Table 1.** Specifications and characteristics of the modeled PVT collector.

### *2.2. Key Performance Indicators*

The performance of such PVT collectors is evaluated using standard key performance indicators. The performance of a collector over a specified period can be quantified using the energy utilization ratio (η*e*), which is defined as below [31]:

$$\eta\_e = \frac{\text{Output energy}\_{\text{electrical}}}{\text{GHI} \times \text{collector area}} + \frac{\text{Output energy}\_{\text{thermal}}}{\text{GHI} \times \text{collector area}} \tag{1}$$

where GHI is global horizontal irradiation (kWh/m<sup>2</sup> ), and the collector area is in m<sup>2</sup> . However, the exergy value of both electricity and heat is different. Electricity can be regarded as pure exergy whereas heat contains some exergy value. To account for this, "energy" is replaced by "exergy", which has the drawback of being somewhat less intuitive. The overall exergy efficiency takes into account the difference of energy grades between heat and electricity and involves a conversion of low-grade thermal energy into the equivalent high-grade electrical energy using the theory of the Carnot cycle. The overall exergy of the PVT (ε*e*) . is defined as following expression:

$$
\varepsilon\_{\varepsilon} = \eta\_{\varepsilon} \eta\_{\text{fl}} + \eta\_{\text{el}}.\tag{2}
$$

Carnot efficiency η*<sup>C</sup>* (%) is defined in the following Equation (3)

$$
\eta\_{\text{C}} = 1 - \frac{T\_{\text{in}}}{T\_{\text{out}}} \tag{3}
$$

where η*th*, η*el*, *Tout*, and *Tin* are thermal efficiency, electrical efficiency, outlet fluid temperature, and inlet fluid temperature, respectively.

NPV is defined as a measurement of cumulative profit calculated by subtracting the present values of cash outflows (including initial cost) from the present values of cash inflows over the PVT collector's lifetime. In this paper, we use NPV to evaluate a single investment to evaluate the acceptability of the project [4]. A positive NPV indicates that the projected earnings generated by a project or investment, exceed the anticipated costs. In general, an investment with a positive NPV will be a profitable one, and the higher NPV means higher benefits. This concept is the basis for the NPV decision rule, which dictates that the only investments that should be made are those with positive NPV values. NPV is calculated using Equation (4) as below:

$$NPV = \sum\_{t=0}^{n-1} \frac{\mathcal{C}F\_t}{\left(1+r\right)^t} - \mathcal{C}\_0 \tag{4}$$

where, *CF<sup>t</sup>* , *r*, *n*, *t*, and *C*0. are the cash flow of particular year (SEK), discount rate, number of years, year of NPV evaluation, and capital cost, respectively. Where, *CFt*,*r*, *n*, *t*, and ܥ are the cash flow of particular year (SEK), discount rate, number of years, year of NPV evaluation, and capital cost, respectively.

*Buildings* **2020**, *10*, x FOR PEER REVIEW 6 of 30

The payback period is the time for a project to break even or recover its initial investment funds, where the cash flow starts to turn positive and can be given as in Equation (5). The payback period is the time for a project to break even or recover its initial investment funds, where the cash flow starts to turn positive and can be given as in Equation (5).

$$PP = T\_{\left(CF\_{l} > 0\right)} \tag{5}$$

#### *2.3. Research Methodology 2.3. Research Methodology*

The simulation is carried using a validated tool developed by the manufacturer of the studied PVT collector. The Abora hybrid simulation tool [32] was used to map the performance across 85 cities shown in Figure 2. The cities were chosen based on population density and geographical coordinates in different countries to represent a large market potential in these regions. A large number of selected locations for analysis are concentrated within Europe, with limited locations in India, United States, and Australia. The selection of locations is also restricted due to the availability of weather and GHI data in the simulation tool. The simulation tool accepts a wide range of design and financial input parameters, e.g., location and weather resources, electrical and thermal demands, local energy tariffs, specific storage volume, PVT panel and installation parameters, interest rate and financing period, etc. The complete list of various inputs used is shown in Table 2. The performance model used in the tool for evaluation of PVT performance is validated in [24], where a heat pump system integrated with 25 PVT modules was monitored, and measurements were also compared with the dynamic simulation model built in TRNSYS for Zaragoza, Spain. This model has observed thermal and electrical performance of collectors is accurate with measured data (4.2% deviation), however, a slightly higher deviation in heat pump performance was noted due to limitations in the black-box model of the heat pump in the studied energy system. The simulation is carried using a validated tool developed by the manufacturer of the studied PVT collector. The Abora hybrid simulation tool [32] was used to map the performance across 85 cities shown in Figure 2. The cities were chosen based on population density and geographical coordinates in different countries to represent a large market potential in these regions. A large number of selected locations for analysis are concentrated within Europe, with limited locations in India, United States, and Australia. The selection of locations is also restricted due to the availability of weather and GHI data in the simulation tool. The simulation tool accepts a wide range of design and financial input parameters, e.g., location and weather resources, electrical and thermal demands, local energy tariffs, specific storage volume, PVT panel and installation parameters, interest rate and financing period, etc. The complete list of various inputs used is shown in Table 2. The performance model used in the tool for evaluation of PVT performance is validated in [24], where a heat pump system integrated with 25 PVT modules was monitored, and measurements were also compared with the dynamic simulation model built in TRNSYS for Zaragoza, Spain. This model has observed thermal and electrical performance of collectors is accurate with measured data (4.2% deviation), however, a slightly higher deviation in heat pump performance was noted due to limitations in the black-box model of the heat pump in the studied energy system.

**Figure 2.** The simulated locations for techno-economic analysis. **Figure 2.** The simulated locations for techno-economic analysis.

This paper further applies the digital numerical map approach based on heat maps to visualize the performance of various indicators across simulated locations. The simulation results for all locations are exported to Microsoft Excel for calculations of energy and exergy efficiency [33]. After this, the results are visualized using QGIS tool, which provides a heat map rendering to design point layer data with a kernel density estimation processing algorithm [34]. Initially, a parametric study of the components at system level is considered according to the operation flow of the simulation tool indicated in the flow chart shown in Figure 3. Then, the simulations are carried with defined boundary conditions and the

results are represented subsequently as monthly electrical and thermal performances, energy savings, economic parameters such as NPV, and payback period. Collector azimuth Financing period models Storage tank volume Interest rate

*Buildings* **2020**, *10*, x FOR PEER REVIEW 7 of 30

**Table 2.** Technical and economic input parameters.

**Technical Parameters Economic Input Parameters** 

margin

components

Type of auxiliary system Material profit margin Number of bedrooms Operation and maintenance

DHW temperature Pricing of all system

Dwellings occupancy Annual maintenance cost Number of collectors Electricity price increment Collector tilt Auxiliary fuel price increment

Type of application (domestic/industrial) Type of mounting structure Type of demand (hot water/space heating) Type of inverter

**Table 2.** Technical and economic input parameters. Meteorological parameters (irradiation/ambient temperature/albedo, etc.) Opening interest rate

**Figure 3.** Operation flow of the simulation tool. **Figure 3.** Operation flow of the simulation tool.

This paper also considers the economic performance of the collector in two different financial models, which are described below: This paper also considers the economic performance of the collector in two different financial models, which are described below:


The economic analysis results highlight the economic parameters, such as NPV and payback period per unit collector area, for all locations. Furthermore, the uncertainty and sensitivity parameters are discussed, and the strategy in decision-making for investing in PVT technology is recommended. The digital mapping method is applied to compile and format the techno-economic The economic analysis results highlight the economic parameters, such as NPV and payback period per unit collector area, for all locations. Furthermore, the uncertainty and sensitivity parameters are discussed, and the strategy in decision-making for investing in PVT technology is recommended. The digital mapping method is applied to compile and format the techno-economic performance data into a virtual image, which aims to produce a general map with KPIs of such a PVT system that gives appropriate representations of the dedicated areas.
