Modeling of the Solar Thermal Energy Use in Urban Areas

Most of the generated electricity in Kosovo is produced from fossil fuel, a part of the energy comes from the import, while participation of renewable resources is symbolic, and a bias between the grid extension and the load of power generated sometimes results in shortage of electricity and thus frequent power cuts. The use of renewable energy and particularly the solar thermal energy represents one of the most promising alternative strategies. In Kosovo, the global horizontal radiation ranges from 1241 kWh/m per year in Shterpce to 1461 kWh/m per year in Gjakova, while the average for Kosovo can be estimated at 1351 kWh/m per year. The average sun duration for the city of Pristine is 5.44 h, while the average horizontal irradiation is 3.79 kWh/m2 per day. Participation of energy consumption in household is still dominant about 41.4% of the total consumption in Kosovo, 15% of this energy is used for domestic hot water. This energy demand can be lowered significantly by using improved building construction techniques and utilization of RESs, especially solar thermal. The first step is to map the city in different areas to locate suitable locations for the installation of solar collectors serving sanitary hot water. The demand for sanitary hot water varies from object to object, this variation depends on whether the building is individual or collective, school institutions or religious buildings, for this reason the classification of buildings was done according to the request for sanitary hot water. After that the demand for sanitary hot water is calculated for several different institutions: Residential houses, Dormitories and Hospitals. For all of the above-mentioned cases the data for: solar fraction, solar contribution, CO2 avoided, collector temperature, financial analysis etc. are gained using the TSOL 2018 software. To evaluate the activation energy for a time period, the daily, monthly and annual performance for three systems which are located in University Clinical Center of Pristine, Kosovo have been analyzed. In addition the results of the mathematical model, simulation and measured solar energy contribution for solar station in Infective disease clinic have been compared. In this paper, a proposal for replacing the conventional water heaters with the domestic solar water heaters (DSWH) is made. A case study for 38289 Residential households in Pristine has been selected. The initial cost of the solar water heater for the city is 60113730 €. The system saves 7274910 € annually and reduced CO2 emission by 22973400 kg. The results from the paper show that the DSWH is economically feasible in Pristine and can result in fuel saving and CO2 emission reduction.


Introduction
Renewable Energy Sources (RES) are the object of ever increasing interest in their use in recent years. The main cause of this interest rise lies in the warning about the depletion of conventional energy sources on the planet: fuel, natural gas, coal and the end of uranium reserves by contrast, RES-s can be considered inexhaustible sources of energy at the human scale, as they use natural energy flows from the sun. Another reason for developing renewable energy sources is the uneven distribution of conventional energy sources on the planet, along with their uneven consumption, This energy demand can be lowered significantly by using improved building construction techniques and utilization of RES-s, especially solar thermal (for: domestic hot water, space heating and industrial heat process), which is not only renewable, but also clean in the sense that the conversion phase does not rise to any greenhouse gas emissions.
Thermal energy sector in Kosovo consists of 4 district heating systems, with an installed capacity considered to be around 332 MW th : This sector has a fairly limited extent locally, fulfilling 3 -5% of total heating demand in Kosovo. District Heating of Termom and Zveçan, due to notorious circumstances, have not responded to requirements for licensing/regulation and monitoring of ERO, thus making impossible to ensure relevant updated data; to this end.
In 2016/2017 season, the entire thermal energy production was from cogeneration plants in TPP Kosovo B, so the activation of boilers with heating oil in DH Termokos was unnecessary [2].
The amount of thermal energy extracted from cogeneration in 2016/2017 season was 225,438 MWh th . The amount of thermal energy received in the heat exchange station in DH Termokos was 221,058 MWh th . It should be noted that this represents an increase of around 13.5% compared to the production of the previous season.
The solar thermal market has an immense potential for growth, in 480GW th on 2018, up from 472GW th a year earlier, while 456 GW th in 2016 [1]. The solar energy received by the Earth during an hour is greater than the total annual energy demand worldwide. It is obvious that we could profit from solar energy to a far greater extent than what is done today.
The biggest problem has been that the cost of solar thermal systems has not been low enough to make these systems competitive on the market. This trend is starting to change with rising energy prices, but it is not changing fast enough. In order to widen and increase the use of solar thermal energy there are several ways to deal with the issue. Legislation is one way, already practiced in some European countries. Governmental subsidies are another way, in which continuity is very important. Although the solar thermal collectors of today are already highly developed it is also important to continue the development of collectors and system designs in order to improve their efficiency, quality, life expectancy, profitability etc.
Despite global development, renewable energy sources are not a universal solution to power supply problems. This is due to some shortcomings. Most important is the lack of power guaranteed by generators, conversion of the main potential of electricity due to the stochastic nature of changes in the primary solar (photovoltaic or thermal energy) and wind sources. In the first case, variations are caused by the day-to-night cycle, through clouds in the sky, or other obstacles between the sun and solar installations. For wind turbines, energy variations are due to wind speed variability, and so on. In this way, the wind power, converted by wind turbines, into mechanical power, has a changeable character because it is proportional to the third-rate wind speed. This disadvantage is much less pronounced in the hydropower plants, due to the presence of water dams and thus prescribing the presence of a certain amount of water in them. In addition, the hydraulic machine's technological experience facilitates the implementation of this renewable energy source. Another serious problem faced by increasing the installed capacity of RES is the initial investment. It is high due to the high price of used materials. In addition to its high price, photovoltaic systems are characterized by low efficiency (less than 15% in real terms), which further limits their distribution, thermal environments in this regard are better, because their efficiency reaches up to 80% [3]. Solar domestic hot water cost in Europe is around Euro 50-160 per MWh of heat, which is usually more expensive than heat from natural gas in urban areas, (Renewable Energy Essentials). Although we will face great challenges, we are sure that we will increasingly use renewable energy sources for the reasons mentioned above (exhaustion of fossil fuels and climate change problem).

Environmental Characteristics and Mapping of Appropriate Surfaces for Installation of Solar Thermal Collectors
This part provides a detailed theoretical analysis of environmental characteristics for the city of Prishtina, as well as did the mapping of appropriate surfaces for installation of solar thermal collectors. Solar Radiation -Pyranometers are instruments for measuring global radiation (direct and diffuse). The following Table 1 clearly shows that the average solar radiation differs from a minimum of 1.38 (kWh/m 2 /day) in December to a maximum of 6.44 (kWh/m 2 /day) in July, but the average is 3.79 (kWh/m 2 /day).  Air Temperaturesis another important factor that is analyzed for the city of Pristine for period 2001-2017 -The distribution of air temperatures in the territory of Kosovo presents a considerable variability. The highest average temperature for a year is in Prizren 12°C, the lowest temperature in Podujevo 9°C, Pristine 11.3°C, while the average annual temperature of Kosovo for year is 9.5°C, Figure 1 presented the minimum, average and maximum air temperature for Pristine city. The mapping of appropriate surfaces for installation of solar thermal collectors. Pristine is the capital city, and the biggest city in Kosovo. Located at coordinates 42°40'0'' North and 21°10'0'' East, Figure 2a). The surface of the Municipality of Pristine is about 523 km 2 . The climate is continental, with cold winters and hot summers, the precipitation average of about 600 mm per year.

Figure 2. a) Map of Kosovo [4]; b) Map of Pristine divided in some areas
To locate suitable locations for the installation of solar collectors serving sanitary hot water, the first step is to divided the city in some areas Figure 2b). The second step is to calculate the roofs area for city of Pristine, which area estimates the space available for solar thermal installation [5]. To estimate how much energy could be generated from the sun on a surface, first the area should be calculated to see how many solar panels could be placed on it. There are various roof tops measuring tools online or software, the software AutoCAD is used Figure 3.

Figure 3. The roof surface which is appropriate for installing solar collectors
To analyze the appropriate roof space for solar collector installation, the division of the city into some areas is made: New Pristine -is an area that is predominantly inhabited by individual housing and very little in multiple residential building single. The surface suitable for installing solar collectors is 86957.54m 2 .
Taslixhja -includes a large part of the city, it is inhabited by single or multiple residential buildings, the houses are so dense that appear as a barrier because some objects are completely in the shade, then many roofs have windows that reduce the appropriate roof surface. The surface suitable for installing solar collectors is 340393.562m 2 .
Shkabaj -this area connects the city of Pristine with Obiliq, has a small surface, in this area there are mainly individual housing, while the roof surface that can be used for solar collector installation is 17041.61 m 2 .
Mati -This area is very well organized, with multiple residential building, but there is also individual housing and can be stated as very suitable for the application of solar energy.The surface suitable for installing solar collectors is 295747.7m 2 .
Arberia-has a big area of the city but is less inhabited, mostly with individual housing. The appropriate surface for installation of solar collectors is 126272.42 m 2 .
Sofali-is the part that has most greener, also in this part we have collective and individual housing. The appropriate surface for installation of solar collectors is 129128.352 m 2 .
KSF Zone -connects Pristina with Fushe Kosova, in this area is Barracks of the Kosovo Security Force, from which it has its name KSF. In this area is also Industrial zone, the town market, part where solar collectors are not recommended. It is very little inhabited, predominantly with individual housing. The appropriate surface for installation of solar collectors is 32249.95 m 2 .
Çagllavica -located on the outskirts of the city, is inhabited with individual housing with house distributed. The appropriate surface for installation of solar collectors is 47511.0034 m 2 .
Kalabria -has single and multiple residential building. The appropriate surface for installation of solar collectors is 103484.204 m 2 .
Kodra e trimave -including a very huge area, in this area mainly have individual housing. The appropriate surface for installation of solar collectors is 305276.95 m 2 .
The city center -dominate the high buildings, multiple residential building, this area includes: the Cathedral, National Library, a part of the University Campus, market, where in these areas is not recommended the application of solar collectors. The appropriate surface for installation of solar collectors is 297403.46 m 2 .
The total appropriate surface for installation of solar collectors for the Prishtina city is 1781467 m 2 .
For all of the selected area it is not necessary to be covered completely with solar collectors [6], knowing that the demand for domestic water depends on the number of inhabitants, institutions (single or multiple residential building, office building, hotel, school, student dormitory, hospital etc) and how much sun its gets based on its geographical location.

Hot Water Demand Calculations and Simulation with T*SOL Software
To calculate the demand for sanitary hot water for several different institutions it is necessary to calculate the monthly average radiation incident on the collector, for this is used the method Isotropic Sky [7]. The following table clearly shows that the monthly average radiation incident on the collector is 4.198 (kWh/m 2 /day), the maximum value is 5.518 (kWh/m 2 /day), in July while the minimum is 2.585 (kWh/m 2 /day), is in December Table 3. The capacity of sunlight is calculated for eight months, since for the other months during the winter, connected the Distring Heating or electricity, which serves for space heating and sanitary hot water.
The average capacity of sunlight for eight months is q=4.746 (kWh/m²). With these data is calculated the daily average sanitary hot water consumption and the number of collectors for some institutions Table 4. Average daily hot water demand in liters per day at 40 to 60°C [8].
The results obtained through these models will then be used as arguments for convincing policymakers or decision makers to make important decisions about investing in solar thermal energy in the city of Prishtina or other cities in Kosovo.

System Definition
This system providing sanitary hot water for Infectious diseases Clinical. In the first step the roof position for the solar collectors was analysed, after that calculated the most suitable surface area for the solar panels Figure 4 shows a) The roof of the clinic while under b) the most suitable part for the installation of solar collectors, the figure shows that there is sufficient space for installation the collectors to meet the demand for sanitary hot water for this Clinic.
Based on the daily demand for sanitary hot water it is calculated the number of collectors that will be placed in this clinic. Figure 4 show that the free surface is 373.42 m 2 ; whereas, according to the calculations before the roof surface that need for installed solar panels is 115.09542 m 2 .

Figure 4. Infectious diseases Clinic
After that was description the main components of the solar hot water heating system for the Infectious diseases Clinic are: Closed-Loop System -The system is designed as a standard closed loop system, where a pump circulates a water/propylene glycol mixture through the collectors and an external heat exchanger.
Infectious Diseases Clinic -A total of nine modules were installed at the Clinic, with two parts: Solar I of four modules plumbed in series. Two set of seven panels are plumbed in parallel and two modules with six panels also plumbed in parallel.
Solar II of five modules plumbed in series, four set of five panels are plumbed in parallel and one module with six panels.
Preheat Tank -There are two preheat tanks in this clinic (with 2×2000 liters-for two parts), that heated water from the solar thermal collectors and preheat the domestic hot water before it is supplied to the auxiliary tanks. As shown in Figure 5, the preheat tanks are heated solely by the collector array. Installed Costs -The installed system cost of the 52-panel system at the Infectious diseases Clinic is 41040 € (for both of them Solar I and Solar II). A photo of the 52 -panel divided in tri part, installation at the Infectious Diseases Clinic is provided in Figure 5.

Figure 5. Schematic Solar Hot Water System at the Infectious diseases
To evaluate the active energy of a system for a time period, it is necessary to analyze the daily, monthly and annual performance for that system, data monitoring for station Solar II during the period May -September are presented in the Figure 6.

Mathematical Model
In this part is given the formulation and development of the mathematical model using the F-chart method to estimate the annual thermal performance of active heating systems for building, where the minimum temperature of energy delivery is near 20°C.
It was designed for a standard water heating system, where the conditions of the simulations were varied over appropriate ranges of parameters of the system design. This makes the f-chart method viable for a very limited range of parameters (Collector area, Tank volume, Tilt angle etc.), otherwise the results obtained with the correlations would be wrong.
In order to obtain results for a solar thermal system, the input data is necessary (Location -Pristine, Latitude -= 42 39 ′ , Slope -45°, and other data shown in Table 6). This model is used for Domestic Water Heating. The two dimensionless groups are: : Monthly average transmittance -absorptance product [7,11].
The overall loss coefficient and the optical performance ( ) are necessary parameters to insert in the numerical model; however these parameters change depending on the efficiency of the solar collector and on the weather data of the solar system.
The Daily Heat load for Infectious Disease Clinic-Solar I is 4000 (l/day) as analyzed before.
In F-chart method, the system's performance is expressed in terms of f, which is the fraction of the heating load supplied by solar energy during each month.
The fraction of the annual water heating load supplied by solar energy is determined by repeating the calculation of X, Y, and f for each month and summing the results as indicated by Equation 4. The Table 7 shows the results of these calculations. In this part the f-chart numerical model will be used to calculate the monthly absorbed solar energy for the year 2018, these results will be compared to data from the T*SOL software. Weather data, monthly average ambient temperature, monthly solar radiation, monthly clarity index, ground reflectance/absorptance, the overall loss coefficient and optical performance previously calculated were also used in the calculations as well as the daily heat load previous are mentioned before. In the following, an annual overview of the Infectious diseases Clinic was made. Annual solar contribution, money saves and C 2 , reduction were calculated as follows: • Annual solar contribution data; • Money saves depend on the fuel, in this case is electricity; • CO 2 reduction. Where: Annual solar contribution f-chart Table 10   The following we can shows the results from monitored for the Infectious diseases Clinic, from 1 May to 20 September, solar contribution 6659 (kWh), annual money saves 483.6537 (€) and annual CO2 reduction 1535.075 (kg).

Comparison between Modeled, Simulation and Measured Solar Energy Contribution
Experimental measurements for this system were realized in period from 01 May to 29 September. Since, this time is considered the time with highs potential Table 10. From month October to March during the less sunny seasons have been approximated, no enough solar, the energy is supplied from District heating.
The Table 10 shows the total annual solar contribution that is simulated with T*SOL is 40960 kWh per year, while the solar contribution calculated with mathematical model is 38397.06 kWh per year, but data from measured with less than mathematical model and simulation, 29602 kWh per year. Mathematical model and T*SOL yielded somewhat higher total solar contribution compared to the measurements, which is mainly explained by the lower solar irradiation the measured year compared to the climatic data used in the simulation.
Based on real data from this clinic, is done the model for solar thermal energy for city of Pristine.

The Model for Solar Thermal Energy Case Study City of Prishtina
Participation of energy consumption in household is still dominant -about 41.4% of the total consumption in Kosovo, 15% of this energy used for domestic hot water. This energy demand can be lowered significantly by using improved building construction techniques and utilization of RES-s, especially solar thermal. Replacing the electrical heaters with solar water heaters could also lead to reducing the peak load demands. The use of clean energy in electricity generation will also lead to the reduction of carbon dioxide emission to some extent. The energy model was developed for a typical house in the Pristine City using the T*SOL Software and compared with F-chart method.
This basic scenario considers a typical house with 5 occupants (the average size of the family is 4.8 occupants) situated at the Pristine city baring alone the full initial cost of the SWH purchase and installation. Other scenarios can also be developed for number of Households in the city. Based in data from the Kosovo Agency of Statistics the number of residential households in Pristine is 38289.
The annual solar contribution is estimated to be 68%, for sanitary hot water temperature 50˚C, whereas the annual total load is calculated to be 3802.77 kWh /year and the annual covered load is equal to 2606.38 kWh/ year. Table 12 shows that the annual solar contribution (based in input data from Table 11) is 2606 kWh, the amount saved by using solar energy for water heating is 189.306 (€). The system reduced C 2 , emission by 600.84 (kg), which is an important amount of reduced C 2 , in the atmosphere. To calculate the demand for sanitary hot water for the city of Pristine, with the number of inhabitants 210282 [13], was taken the total of residential household in Pristine with around 38289 units, and the average number of people per house 5 occupants. The data for the SWHs for Pristine city is summarized in Table 13. The initial cost of the solar water heater is 1570 (€).The maintenance cost is considered to be 5% of the initial investment.  Table 13 shows that the annual solar contribution for city the Pristine is 99781134 kWh, to arrive this request is required to be installed 76578 collectors with total gross surface 186084.5 (based in calculation in chapter three: the appropriate surface for installation of solar collectors for city Pristine is 1781467 m 2 , which means that we have enough space). The initial cost of the solar water heater for the city is 60113730 €.The system saves the annual money by 7274910 € and reduced C 2 , emission by 22973400 kg, which is an important amount of reduced C 2 , in the atmosphere. In the following, the result for cumulative cash flow analysis for 20 years was presented [14][15][16][17][18][19]. The financial characteristics of the solar water heating system investigated results in savings equal to 71343894 (€) over its life time with a payback period equal to 9.6 years, see Figure 8. These characteristics can motivate the government or municipality of Pristine to invest in solar water heating system since they have a payback period 9 years, they can be easily installed in either flat or slope roof, they have low maintenance cost (approximately 1-4% of the investment cost) as well as positive net present value.

Conclusions
What is the most important conclusion to be drawn from this project?
We know that we didn't solve the mystery of the modest dissemination and implementation of solar thermal. Application of solar thermal energy in Kosovo nowadays is very symbolic. The first solar collectors were installed during 2008-2009.
But we managed to partly widen the system boundaries and method basis in the study of solar thermal especially for domestic hot water, which seems to be a prerequisite to understand the complexity of the dissemination process and identify barriers that tend to appear in the different implementation phases. Based on the result from mathematical model, experimental part, and simulation with T*SOL software, we can conclude that: altogether with a good technological base, creates favorable conditions for the exploitation of solar energy in Kosovo.

Project result overview
The aim of this project was to study a solar domestic water heating system, and replacing the electric water heating system with solar water heating system for city of Prishtina is proposed, to develop a numerical model based on the fchart method and to compare results obtained from the model with data obtained from T*SOL software and experimental data.
The work starts with an introduction on solar energy, in Kosovo around 93% of the primary energy is provided by fossil fuels, while participation of renewable resources is symbolic. Participation of energy consumption in household is still dominant -about 41.4% of the total consumption in Kosovo, 15% of this energy used for domestic hot water. This energy demand can be lowered significantly by using improved building construction techniques and utilization of RES-s, especially solar thermal.
To provide a more complete support to design the system for city of Prishtina, finance, install and utilize solar energy for sanitary hot water from solar energy, besides other things, are required to possess data with the following information: solar radiation on the optimum horizontal and sloping (tilt) plains for the specific area / location where solar panels will be installed, other climate conditions of the region / location, including average temperatures of air, water, etc., Based in Prishtina's meteorological data:  Prishtina located at coordinates 42°40'0'' North and 21°10'0'' East;  The average sun duration for city of Prishtina is 5.44 h;  The average horizontal irradiation is 3.79 kWh /m²/day;  The average temperature in July as the hottest month is 19.2°C, while January as the coldest month has a mean value of -1.3°C;  Annual optimum angle is 33 degree.
To locate suitable locations for the installation of solar collectors serving sanitary hot water, the first step is to divide the city in some areas, and after that, to calculate appropriate roof space the software AutoCAD is used.
 Appropriate surface for the city is 1781467 m 2 .
But, for all of the selected area it is not necessary to be covered completely with solar collectors, knowing that the demand for domestic water depends on the number of inhabitants, institutions (single or multiple residential building, office building, hotel, school, student dormitory, hospital etc.) and how much sun is gathered based on its geographical location.
It is clear that the demand for sanitary hot water varies from object to object. In some sectors, sanitary hot water is necessary for example in residential buildings, while in others it increases the quality of life, for example in medical education institutions etc. Another important factor is to calculate monthly and yearly average total radiation on a tilted surface for the city. For this calculation based on the Duffie and Beckman (2013) [7], mathematical software MathCAD has been used:  The monthly average radiation incident on the collector is 4.198 kWh/m 2 /day, After that, the demand for sanitary hot water for several different institutions has been calculated, in different points of the city: residential house, building, dormitory and two clinics. For these institutions for simulation is used T*SOL software that allows to accurately calculate the yield of a solar thermal system dynamically over the annual cycle.
As a general conclusion it can be suggested that for sanitary hot water temperature from 45 to 60˚C:  The average total solar fraction for Pristine is 58%;  The average solar contribution is 605 kWh/m 2 ;  The total annual savings for CO 2 emissions is 69127.5 kg;  The average Savings District heating is 4355.96 kWh.
To analyze in detail the solar contribution, are described data for the main components for three systems,these systems serves for sanitary hot water in two Clinics at The University of Clinical Center of Pristine. For both cases are presented, the monthly analysis starting from May to September month, because this time of period is the time when the solar system reaches the highest energy potential, compared to the other months of the year.
Based on the gained values for Solar Hot Water System at the Infectious diseases Clinic, Solar I is seen that the maximum value is around 12-13 o'clock, the highest recorded value is 30000 Wh in July, in this system usually at night from 19 to 9 o'clock there is no energy exchange. After the comparison the data recorded by the controller from Solar station II in Infectious diseases Clinic and the data obtained with the T*SOL Software for daily solar contribution. The analysis shows a slight difference of solar contribution between the values recorded in this station and the values obtained by simulating with TSOL software. The main reason is the weather data for 2018 year, where it is known that this year especially July had lower temperatures compared to what T*SOL receives from Meteonorm software.
The mathematical model also developed for Infectious diseases Clinic -Solar II based on the f-chart method by Duffie and Beckman (2013) [7]. For sanitary hot water temperature 45˚C average total solar fraction for this station is 64.9%, the annual solar contribution is 38397.06 kWh. Then the T*SOL software was used to compare data for solar water heating system: between mathematical model and data from software, this difference for solar fraction is 0.05%. Based in the data from mathematical model, for annual solar contribution 38397.06 kWh is calculated:  The money saved 2788.839 €;  The C 2 , reduction 8851.533 kg.
Based on the results for annual solar contribution and annual financial savings was reached the result for cumulative cash flow analysis. The simple and equity payback periods are 8.6 years. The investigation for replacing the electrical heaters with solar water heaters for city of Prishtina was based in terms of the electricity cost, capital cost, maintenance cost and the CO 2 emission. The energy model was developed for a typical house with 5 occupants in the Prishtina City using the T*SOL Software and compared with F-chart method. Based on results of the numerical analysis:  The annual solar contribution for this model is 2606 kWh;  The amount saved by using solar energy for water heating is 189.306 €;  The system reduced C 2 , emission by 600.84 kg.
For the life span of the solar water heater 20 years, annual solar contribution 2606 kWh and annual savings of 189.3 € was reached the result for cumulative cash flow analysis, the payback period is equal to 9.6 years. To calculate the demand for sanitary hot water for the city of Prishtina, the total of number of residential households in Pristine with around 38289 units and average number of people per house 5 occupants has been considered.
Based on the calculation the annual solar contribution for city of the Pristine is 99781134 kWh, to cover this request 76578 collectors should be installed. The initial cost of the solar water heater for the city is 60113730 €, the payback period is equal to 9.6 years. The annual savings of the system are 7274910 € and reduced C 2 , emission by 22973400 kg, which is an important amount of reduced C 2 , in the atmosphere. As suggested, if solar water system is to be installed, first step is simulate the system using the software with default conditions. The computing time will be very small, but we will have very accurate data about the solar contribution, solar fraction, CO 2 emissions, financial analysis etc.
The results obtained from this investigate highlight the positive aspects of the use of solar energy to heat domestic sanitary water. On one hand are the environmental aspect, the CO 2 reduction are very important, the use of traditional fossil fuel is avoided, hence the emission of not only carbon dioxide, but also methane nitrous oxide and so on. On the other hand, is the economical aspect, the savings made from the studied solar system are not that high, the payback time is 9.6 year. But Prishtina is ranked among the most polluted sites and this is mainly: by transport, old power plants, and fossil fuel use as a heating fuel for winter seasons.
Therefore, Kosovo needs to look to maximally exploit renewable energy sources for this risen the money saves are not that important, sometimes, in order to get a better environment for our life, the economical profits should be considered as a secondary factor. It is imperative to try and globalize the use of renewable energies for the maximum that we can, using solar energy for sanitary water heating is not a very profitable way but it is surely extremely useful for our environment. The SWHs system is the possible solution because Kosovo struggles to satisfy the energy demands and it is difficult because we have two very older power plants.

Conflicts of Interest
The authors declare no conflict of interest.