What is the greatest problem in the Polish energy sector is the sustained, growing cost trend (OECD, 2020; Ministry of State Assets, 2019; Kaplan and Kopacz, 2020; Ministry of Culture and Environment, 2020; Tomasik et al., 2018; Weather data and software for solar power investments, 2020; NFEPWM, 2020). With energy prices frozen by law in 2019 in Poland (at the level of June 2018) and constantly rising costs, it will not be possible to avoid high price increases in 2021. The further increase in energy costs in Poland and prices (within 2–3 years) will have to be shared by all energy consumers, including households, but also industry, farmers and public utilities. These regulations apply to the entire territory of Poland (CIRE.PL, 2020; Janoś, 2020). Furthermore, this thesis is confirmed by the high electricity prices in Poland as compared to other EU countries. As it stems from Eurostat data, among Member States of the European Union, electricity prices for households were the highest in the second half of 2019 in the following Member States: Denmark (0.2924 EUR/kWh), Germany (0.2873 EUR/kWh), and Belgium (0.2860 EUR/kWh), while the lowest electricity prices were recorded in Bulgaria (0.0958 EUR/kWh) and Hungary (0.1097 EUR/kWh). Poland, with the average rate of 0.1376 EUR/kWh, is one of the countries in the middle of this list (Eurostat Statistics Explained, 2020). It may be observed that from year to year, total prices have fallen in 10 EU Member States, including Poland, where tax reductions are relating to the installation of renewable energy sources. A similar downward trend may be observed in Poland for non-household consumers, which is another argument for investing in the RES and, in particular, solar collectors or panels PV.

The argument for investing in panels PV or solar collectors installations in Poland is that solar radiation provides a number of benefits, such as lightweight design, no emissions or noise, no need for constant operation, and the ability to operate in different weather conditions. What has to be considered before making a decision on which installation to choose is the cost of their construction, durability, and efficiency. These criteria should be the basis for the selection, from a range of models available on the market, offered by global manufacturers, of solar collectors that will provide 70% of the desired energy on average during the year and 100% during the summer months.

Although both solutions differ significantly in structure, operation, and type of obtained energy, sometimes, they are confusing. Solar collectors and photovoltaic panels are two systems that enable the acquisition and processing of solar energy. Panels (photovoltaic modules) are used to convert the energy of solar radiation into electricity. The energy obtained in this way has a wide range of applications. It can be used to illuminate the building, as well as power electrical and electronic devices. In turn, solar collectors convert the energy of solar radiation into thermal energy. In this way, the collectors can heat domestic water, support the operation of central heating, or serve to heat water in a swimming pool. Both solar collectors and photovoltaic panels are energy-saving solutions, but the profitability of investments of each solar system should be considered in the context of their use.

The solar collector market in Poland has a huge but so far highly untapped development potential. At the end of 2018, the total capacity of the RES installations connected to the power grids, as identified by the Office for Energy Regulation (Kowalski, 2020), was 8,593 MW, and the solar collectors’ sources accounted for only 1.7% of licensed sources, but after 5 years of incubation in a niche, the solar collector technology has a chance to become the second (after onshore wind power) RES technology in Poland within a few years.

In 2025, the total installed capacity in solar collectors may reach 7.8 GW, which means that already in 2025, the capacity of solar collectors sources will exceed the capacity assumed in the National Energy and Climate Plan for 2030 (CIRE.PL, 2020).

The observed and forecast growth of the market of installed solar energy is largely possible thanks to subsidies in the form of government programs: “My Electricity,” “Transparent Air,” “Energy Plus,” and “Agroenergy,” and the change in regulations allowing households to engage in energy generation and feed into grid made in 2016 (The Sejm of the Republic of Poland, 2019), which creates the possibility of a large interest in the production of green energy.

Thanks to the support of government, EU programs, and the amendment of the national law, the RES industry was the only industry that was able to mobilize more capital for investment between 2019 and 2020 than the entire conventional energy sector. This industry is also able to win the confidence of Polish and foreign capital investors, which is confirmed by the stock exchange index IEO (Ministry of Development, 2020), presented in Figure 1.

As a result of analyzing the extremely dynamic solar collector market, the Institute of Renewable Energy developed the first, copyright stock exchange indicator called IEO. The indicator includes the listings of solar collector companies operating on NewConnect and on the main floor of the Warsaw Stock Exchange. It is a simple and transparent indicator of the economic situation of the solar collector market in Poland.

The purpose of the research was to determine the return on investment of using solar collectors in a single-family residential building for one calendar year, with local weather conditions, as exemplified by Cracow in Poland. On the basis of the collected data and the adopted assumptions, calculations were made to estimate the amount of energy that may be obtained in the form of heat. A single-family house of the area of 150 m² was subjected to economic analysis. The demand for thermal energy is 16,000 kWh, including 6,000 kWh for hot water preparation and 10,000 kWh for heating. Hot water temperature: t_w = 50°C, cold water temperature: t_z = 7°C. Radiation conditions, reaching 1,000 kWh/m² per year with the average annual insolation of 1,600 h, are favorable. In addition, the solar collectors will be installed on the roof, positioned to the south, and the angle of inclination to the surface will be 45°. The calculations will be made for a vacuum collector of the actual absorption area A ≈ 6.5 m², efficiency η = 0.8, and daily output of 300 dm³. The number of vacuum tubes is 20, the tube lengths are 1,800 mm, and the diameter is 58 mm. The height of the collector is 1,975 mm and its weight is 93 kg, P_o = 4.5 kW.

These parameters are taken from the technical specifications of individual devices, technical standards, and national requirements for the installation of solar collectors in Poland. Parameters are averaged for a given roof area and building volume. When making similar calculations, one should take into account the amount of insolation that varies in a given territory of Poland and has an impact on the return period of investment in a given solar installation.

On the basis of the collected data and the adopted assumptions, calculations were made to estimate the amount of energy that may be obtained in the form of heat, depending on the absorber area. The amount of heat, adjusted appropriately taking into account the existing demand for heat, allows determination of the NPV, assuming the price of energy from the replaced heat source. Next, the internal rate of return (IRR) was calculated (Olczak et al., 2018; Zdyb and Gulkowski, 2020): NPV = ∑ i = 1 n NCF i ( 1 + k ) n = ∑ i = 1 n P i ( 1 + k ) n − ∑ i = 1 n N i ( 1 + k ) n , (1) where CF_i—net cash flow expected in year i (i = 1, 2…n), N—initial investment outlay, k—appropriate market capitalization rate, n—the lifetime of the project effect in years, NCFi—net cash flows, P_i—revenue of the year, and N_i—expenditure of the year. IRR = i 1 +   P V * ( i 2 −  i 1 ) PV + | NV | , (2) where IRR—internal rate of return, i₁—interest rate, NPV > 0, i₂—interest rate, NPV < 0, PV—NPV calculated in accordance with i 1 , and NV—NPV calculated in accordance with i 2 .

The data used for the calculation are derived from measurements made on an actual solar installation, which allows comparison of the results with other installations. The obtained heat yield Ustj was referred to the radiation for an average year. In addition, the calculations were made with the use of the radiation model according to the following formula (Olczak et al., 2020a; Zamasz et al., 2020): U stj = ∑ y ( U njy ⋅ I ptj / I njy ) 3 , (3) where U_nyjn—measured solar yield in month j, year y, kWh/month; I_ptj—insolation on a flat surface by average year in month j, kWh/(square meter × month); and I_njy—measured solar radiation of the flat surface in month j, year y, kWh/(square meter × month), y—measurement years.

Since the insolation of Ijrw is a stochastic variable, in the further calculations, the insolation was assumed on the basis of the value drawn with the use of the Monte Carlo method (CSO, 2019; Kryzia et al., 2020; Kryzia et al., 2020). The model of statistical deviations of solar yields used in the simulation describes relationship (2). The simulation allowed adjustment of the average solar yields to the average insolation and its standard deviation over the years, taking into account the amount of the heat demand. I j r w ( β , γ ) = f ( s d ( I p t j ) , I p t j ) , (4) where I_ptj—insolation of a flat surface in month j of a typical meteorological year, kWh/(m² × months); sd (I_ptj)—standard deviation of the insolation from that in a typical meteorological year in month j, kWh/(m² × months).

In order to improve the economic effect, it was proposed to increase the area of the absorbers. Furthermore, on the basis of actual measurements from this installation, reflecting the influence of many nonmeasurable factors on the efficiency of solar energy conversion, simulations of the economic effect were made for different heat demand volumes. The obtained results have been generalized, which allows their usage in the process of selecting the size of collector area for similar installations.

One of the advantages of building solar collectors installations is the ability to reduce greenhouse gas emissions. The analysis considered two factors, the benefit of reducing CO₂ emissions from the combustion of fossil fuels energy electricity emission factor in Poland, kg_CO2/kWh (EECO₂), and CO₂ emissions that arise during the production cycle of solar collector CO₂ emission factor for solar collectors, kg_CO2/kWh (EFSCCO₂). In Poland, the CO₂ emission factor per kWh for end users of electricity (according to KOBiZE) is 0.765 kgCO₂/kWh. Since the emission factor depends on the type of fuel (brown coal, hard coal), the analysis of EECO₂ has been carried out for a range of 0.4–1 kg _CO2/kWh. It has been assumed that during solar collector production, the EFSCCO₂ changes in the range of 20–85 gCO₂/kWh (Ren et al., 2020; Louwen et al., 2016; Mohammadi et al., 2018; Jäger-Waldau et al., 2020; National Center for Emissions Management, 2020). A C O 2 = E A G ⋅ ( E E C O 2 − E F S C C O 2 ) , (5) where ACO₂—the amount of CO₂ emission avoided, Mg/year; EAG—annual energy gains, kWh/year; EECO₂—energy electricity emission factor in Poland, kgCO₂/kWh; and EPVCO₂—CO₂ emission factor for PV, kgCO₂/kWh.

Of course, reducing CO₂ emissions is a secondary issue and has no impact on the return on the planned investment. However, it should be emphasized that solar installations can be used as an ecological potential source of energy production. Thanks to this, we contribute to reducing the emission of electricity and heat generation.

The investment outlays relating to the extension of the system for the preparation of hot water with a solar system for the analyzed building were made in accordance with the methodology presented below.

The energy produced by a solar collector during the year is calculated with the use of the following formula: Q k = G ⋅ A ⋅ η , (6) where G—annual radiation, A—absorber surface, and η—collector efficiency. Q k = 1000   k W h / m 2 ⋅ 6,5 m 2 ⋅ 0,8 = 5200   k W h . (7)

The heat loss in the supply lines and exchanger is ca. 10%, or 520 kWh. The amount of electricity supply required for the system (pump and controller) is ca. 100 kWh. The net energy of the system is as follows: Q u = 5200   k W h − 620   k W h = 4580   k W h . (8)

In order to heat 300 dm³ of water to 50°C, there must be supplied the heat energy of the following value: Q w = m ⋅ C p ⋅ Δ T = 300 ⋅ 0,0011 ⋅ ( 50 − 7 ) = 14,19   k W h . (9)

Assuming the heat loss of 15% in the system, the amount of thermal energy to be supplied to the system during the day is obtained as follows: Q c = 14,19   k W h + 15 % = 16,32   k W h , (10) that is, the annual energy consumption will be: Q r = 16,32 ⋅ 365 = 5956   k W h . (11)

For the collector to be able to heat 300 dm³ of water to 50°C on average per year, it has to work over the following time: Q c = P o ⋅ t → t = Q c P o = 16,32 4,5 ≈ 3,6   h / d a y . (12)

Assuming that the amount of energy in the daytime during the whole year changes within the range of 0–5 kWh/m², at full insolation, the power of the collector will be as follows: P k = 5   k W / m 2 ⋅ 6,5   m 2 = 32,5   k W , (13) and the time required to heat up 300 dm³ will be as follows: t = Q c P k = 16,32 32,5 ≈ 0,5 h . (14)

In this case, the efficiency of the system will be as follows: η = Q u Q r = 4580 5956 ≈ 0,77. (15)

The calculation shows that the average annual savings that may be achieved when heating water with the use of solar collectors amount to EUR 550: 4580   k W h ⋅ 0,12   € / k W h ≈ 550 € / y e a r . (16)

The main factor influencing the degree of return on investment of the solar collectors application is its purchase price—the lower the price, the quicker rate of return (with similar equipment efficiency) (Burgio et al., 2020). In the domestic market, there are representatives of many companies specializing in solar collectors. These systems differ in quality, technical specification, price, and efficiency (Zdyb and Gulkowski, 2020). The system selected for this particular analysis is a system with two panels of vacuum collectors with a collector area of 6.5 m² and the final price of 4,500 EUR. The costs connected with the operation of the system are assumed to reach EUR 50. Taking into account the investment and operating costs as well as profit for different conventional energy carriers, the payback time presented in Tables 2 and 3 was obtained.

The investment related to the use of solar collectors for single-family buildings will pay for itself after 8 years. Taking into account the average service life of the collectors at the level of 20–25 years, the investment is profitable because already in the tenth year of operation, it will bring tangible profits. What also affects the investment return on investment is the area of the absorber used for calculations (Żołądek et al., 2019). On the basis of the collected data and the adopted assumptions, calculations were made to estimate the amount of energy that may be obtained in the form of heat, depending on the absorber area. The amount of heat, adjusted appropriately taking into account the existing demand for heat, allows to determine the NPV, assuming the price of energy from the replaced heat source. Next, the internal rate of return (IRR) was calculated. Figure 4 shows that the annual solar yields increase proportionally to the area of the absorber. Within the range of 2–5 m², they reach 800 kWh and 1,800 kWh, respectively, for the extreme values. In contrast, they differ by 600 kWh between 6 and 12 m².

On the basis of the obtained results concerning heat yields, the applicable heat price, and fixed and variable costs, NPVs may be calculated as a function of the absorber area, in accordance with the following formula and the internal rate of return (IRR). For the analyzed installation, the investment in the extension of the collector area in accordance with the NPV is economically effective for the absorber area within the range of 5.6–7.6 m² and reaches the maximum value for the absorber area of 6.6 m², while the absorber area of more than 7 m² contributes to reducing the value of economic return on investment measures. For example, assuming that the heat demand for preparation of Q of hot water is 6,000 kWh/year in a single-family house and the area of the absorbers is 4 m², then the NPV is negative (−533 EUR). Changing the area of absorbers to 6 m² will result in changing the NPV by EUR 778 as compared with the current NPV (and will amount to EUR 245). The calculations are based on the assumption that the water is heated directly in the absorber of the solar panel. Furthermore, on the basis of map charts that may be drawn up for any place in Poland, the user may select the most economically effective area of the collector absorber, taking into account the user’s heat demand for hot water preparation. Figure 5 shows a map of the effective solar yields as a function of the absorber surface.

Extending the water heating system with a solar system is economically justified (in the event of similar technological solutions) if the annual heat demand is greater than 3,000 kWh. The minimum area of the absorber for a family of five should not reach less than 5 m². What significantly impacts the economic efficiency is the investment cost, which may vary considerably depending on the level of cofinancing. Furthermore, the amendment of the RES Act of 2019 assumes the following:

• the prosumer may use the power grid as an energy storage, provided that he/she has signed a comprehensive contract for the provision of electricity distribution and sale services;

The analysis of ecological effects was carried out on the basis of the results of calculations of the amount of energy obtained from solar collectors installed in a single-family house. The characteristics of the house, the results of calculations of the domestic water heat demand, and the parameters of the selected collectors are presented in Return on Investment Analyses. The basis for calculating the amount of saved fuel and the related amount of reduction of emissions of combustion products was the determined energy demand for heating utility water. The amount of emission reduction depends on the type of fuel saved and the parameters of the conventional heating device. To calculate air pollutant emissions, the index methodology was used, using the results of own calculations of energy and the annual energy yield from vacuum collectors selected for the analyzed house with a total area 6.5 m². Figure 6 shows the results of one of the exemplary graphs of reducing carbon dioxide emissions as a result of the use of solar collectors for the preparation of domestic hot water.

Similar results were obtained for sulfur dioxide, hydrocarbons, dust, nitrous oxide, methane, non-methane volatile organic compounds, and nitrous oxide.

In Poland, with the use of solar collectors in the installation for preparing domestic hot water, it involves reducing the amount of fuel burned, that is, reducing the emission of pollutants into the atmosphere. The ecological effect depends on the type of fuel that has been replaced with energy from solar radiation and is proportional to the amount of energy obtained. The greatest reduction in emissions is associated with the installation of solar collectors in a system based on a coal-fired boiler.

The analyzed installation produced 694 kWh of electricity in July and 6,756 kWh per year, which allows the prevention of CO₂ emissions at the level of 344–410 kg, which will be between 2.4–3.6 Mg per year.

The cost of reducing CO₂ emissions depends on the share of energy sources in the structure of its production. An exemplary emission factor for electricity generated in a gas power plant is 400 kg CO₂/MWh, while for a coal power plant, it is 850 kg CO₂/MWh [59]. Forecasts for the development of the energy sector in the 2060 perspective are varied (Olczak et al., 2020a; PARP, 2020). Depending on the variant, depending on the costs of emission allowances, gas prices, or the share of renewable energy, the share of coal energy in 2020–2040 changes from 62–56% for the variant with high costs of emission allowances from 51–14% with the possibility of reducing the emission level to 85 million Mg CO₂.

With the current emission factors related to the production of solar collectors and the factor related to CO₂ emissions related to electricity production in Poland of 765 g CO₂/kWh, the unit cost of CO₂ emission reduction is approximately EUR 50/Mg CO₂.

In fact, the ecological gains will be higher when the minimum, guaranteed by the manufacturer, energy yield from 1 m² of the solar collector, and the nominal thermal efficiency of the boilers, which are often not achieved under reduced load conditions outside the heating season, are used. The ecological effect shown in this study for a single house may seem insignificant. On the national scale, however, these are potentially significant amounts of reduction of pollutants emitted to the atmosphere.