2. Available Renewable Energy Technologies

-

photovoltaic modules;

-

solar collectors;

-

wind turbines;

-

water turbines;

-

biomass units;

-

heat pumps.

2.1. Photovoltaic Systems

When considering the world’s energy production in 2021 from renewable energy sources, photovoltaic installations rank third in terms of installed capacity ^[33][36]. Recent reports show that this value has increased sevenfold between 2010 and 2020 indicating very dynamic growth and suggesting ever-increasing installed capacity in the future. The very high popularity and affordability of this technology are contributing to its continuous development and thus increasing its efficiency. Photovoltaic installations are very often used as a key energy source in small HRES ^[34][37]. The basic solution is to use standard first-generation modules and place them on the available surfaces of the utility being powered. When planning even the most basic photovoltaic installation, it is important to keep in mind several factors that affect its operation. This directly includes the geographic location of the facility being powered, and thus the angle of the modules from the floor [13]. The efficiency of an installation located in Scandinavian countries will be different from one in the vicinity of the equator. The difference is primarily due to the different values of average horizontal irradiation in these locations, but also the average annual air temperature. The higher it is, the lower the efficiency of PV cells ^[35][38]. This problem is being addressed by the latest research on module cooling systems to enhance their performance. Praveenkumar et al. ^[36][39] showed that it is possible to integrate a PV module with a heat pipe. The research indicated that this decreased the module’s surface temperature by an average of 6.72 °C on a sunny day. The temperature reduction resulted in an increase in cell efficiency of about 2.98% over the comparison module. A different approach was followed by Sornek et al. ^[37][40] in designing a system to cool a PV module by spraying its surface with water. Their research showed that this procedure allowed an increase in the maximum instantaneous power of the modules by about 10% compared to the comparative system. A PV system can be placed not only on the roof or ground—a very popular approach in Asia ^[38][41] is floating PV systems. This solution not only saves space for the installation by using a body of water but also increases energy production by cooling the PV panels ^[39][42]. Studies ^[40][43] show that a floating installation has a lower average surface temperature of the modules by 2–4% compared to an installation placed on the ground. In addition, the authors pointed out that this method of installation reduces the intensity of water evaporation in the water tank used. In recent years, an increasing emphasis on research on photovoltaic cells of the second, third, or fourth generation can be observed ^[41][44]. However, from the point of view of commercial applications, they are characterized in most cases by lower efficiency than first-generation solutions except for multijunction cells, the cost of which prevents their cost-effective use for most small-scale polygeneration systems. Attention should also be paid to technology using focused solar radiation. Using mirrors or lenses, it is possible to increase the efficiency of electricity production from a photovoltaic cell. This technology is called concentrated photovoltaics (CPV). However, a review of this technology ^[42][45] indicates that this solution is not valid for receiving only electricity, due to the occurrence of excessive temperatures. It is advisable to additionally dissipate effectively or use the heat produced. When considering the operation of a statistical PV system, the largest energy yield losses are associated with the shading or fouling of individual PV cells ^[43][46]. This is due to the characteristics of connecting modules in series with each other in a classical solution. This results in small values of current with voltage increase, which positively affects the safety of the installation and limits the ohmic losses on the solar cables. In order to make the entire photovoltaic circuit operate at uniform current parameters, a DC/DC voltage converter is present at each PV installation. It is designed to track the maximum power point of the circuit and adjust the installation voltage to achieve it. This component is called a maximum power point tracker (MPPT). With this configuration, partial shading of the installation or even of a few cells of one module negatively affects the performance of the entire installation. To cope with this problem, more and more research is being dedicated to the proper configuration of MPPT control. There are many options for tracking the point of maximum power ^[44][45][47,48], including: traditional—based on direct measurements of voltage and current; mathematical modeling—showing the locations of the point of maximum power; and intelligent algorithms—allowing the creation of a neural network that is taught to look for the right parameters to obtain the best results. Photovoltaic installations have plenty of advantages that demonstrate their versatility of use in small-scale polygeneration systems. These include, first and foremost, the relative stability of operation under conditions of adequate insolation and low price.

2.2. Solar Collectors

The abovementioned photovoltaic modules are not the only way to use solar energy. Thermal solar collectors are also used for this purpose. The most common are classic flat-plate collectors, which have a layer that absorbs solar energy and a piping in which the working medium is heated. The heat is then transferred usually to a domestic water circuit. In addition to flat-plate collectors, tubular vacuum variants, which use heat pipes that allow the heating medium to move, are also popular. The adoption of a vacuum allows the absorber surface to be well insulated, resulting in less heat loss to the environment. It is possible to integrate a solar collector with a photovoltaic module, creating a photovoltaic–thermal (PV/T) hybrid system ^[46][49], which allows not only the supply of electricity but also heat. The hybrid cogeneration approach to energy production increases the efficiency of the entire system. However, this solution requires suitable climatic conditions for profitable operation. Tracking the performance of existing installations ^[47][50] shows that, depending on the latitude, the thermal or photovoltaic part of the hybrid module has better performance. In addition, using such a system to heat water provides temperatures ranging from 40 °C to 60 °C, which typically limits the possibilities of its use in relatively low-temperature applications ^[48][51]. In the case of photovoltaic cells, the concentration of solar radiation is associated with large losses due to excessive temperature. For solar power systems, however, the possibility of achieving relatively high temperatures is key. Several types of collectors using concentrated solar radiation can be distinguished ^[49][52]: compound parabolic collectors ^[50][53], trough parabolic collectors ^[51][54], parabolic dishes ^[52][53][55,56], Fresnel lenses ^[54][57], and power towers ^[55][58]. Each of these differs in a number of factors, but comparisons should mainly be made on the basis of operating temperature, from which the appropriate solution can be selected to meet needs. Collectors operating on concentrated solar radiation are not often used with small-scale systems, so can be observed as large-scale solar farms. In order to improve the performance of a collector system, there is a lot of research involving the proper adjustment of the water circulation system. This can be carried out in a direct way, i.e., the heating medium in the collector is water, which when heated flows directly into the building pipelines. The second option is a system in which a separate heating medium is present, flowing in a separate circuit near the collector and transferring energy to a second circuit with domestic water. The movement of the medium is typically assisted by circulating pumps and is rarely achieved by natural convection, taking advantage of the difference in fluid density at different temperatures. Optimization of the system can also involve selecting the most efficient heating medium ^[56][57][59,60] or locating heat storage. In most solar collector systems, tanks can be observed inside the building; however, especially on a small scale, external tanks sometimes integrated into the collector itself are popular. In countries with high temperatures, this can allow additional reheating of the water in such a tank ^[58][61]. Collector systems can serve as the main source of hot water in a hybrid system, but they are increasingly encountered at the same time as auxiliaries, which are mainly intended to reheat the circulating medium to improve the performance of the entire system ^[59][62]. These are not the most popular methods of obtaining thermal energy; however, under the right environmental circumstances, they can be a very good source of energy for small-scale polygeneration systems or as the main source of heat in large-scale systems ^[60][63].

2.3. Wind Turbines

Wind energy is the second-largest RES in the world ^[61][64]. Wind turbines are mainly used to generate energy from this source, and when considering this technology, the first classification points to turbines with vertical (VAWT) and horizontal (HAWT) axes of rotation. The first group can be divided into two basic types ^[62][65]. The first is the thrust type ^[63][66], which includes the Savonius rotor [5] and Sistan rotor ^[64][67], where blades are usually bowl-shaped. The rotation of turbines of this type is caused directly by the force of wind pressure. They are characterized by the lowest rotational speed and therefore the lowest power yield. The second type consists of turbines based on lift force, which requires the use of blades in the shape of airfoils, and this group can include the Darrieus turbine ^[65][68] or H-rotor ^[66][69]. They are characterized by a higher rotational speed, which is able to match or even exceed turbines with a horizontal rotation axis. Unfortunately, the big problem of this solution is the relatively high wind speed required to start operation. Nevertheless, it is possible to combine both types to reduce its value ^[67][70]. Due to their design, VAWTs always generate negative torque, which reduces their efficiency. One way to deal with negative torques is to use augmentation methods, which allow changing the direction, to a certain extent, of the wind flowing through the turbine ^[68][71]. The scientific literature indicates that it is possible to improve the efficiency of turbines with a vertical axis of rotation using elements that increase wind speed ^[62][65]. These could be a deflector plane or a diffuser using the Venturi effect. Also very promising are the results of studies of the usage of methods such as air currents near buildings ^[69][72] or suitably shaped terrain ^[70][73]. These methods allow wind turbines to be placed in urban centers or next to individual homes, demonstrating the possibilities of their usage in distributed energy. The possibilities are supported by the fact that VAWTs take up less space than HAWTs. As concerns VAWT design details, Qiang Gao et al. ^[71][74] showed a novel approach to controlling the blade arrangement in a Darrieus turbine, which can also positively affect its operation. In the case of lift-based types, it is very important to choose the right shape of airfoils for the blades ^[72][75], which is also a vital aspect of designing turbines with a horizontal axis of rotation ^[71][74]. Because of their higher efficiency, HAWTs are the most popular among commercial applications. A significant number of wind farms are based on this technology. In addition to dimensions, they can differ in the number of rotor blades. The best choice turns out to be a three-blade turbine. It allows very high efficiency while minimizing noise and manufacturing costs. Considering a different number of blades, the performance of the turbines is close ^[73][76], with the single-blade rotor, which by shifting the center of gravity falls into a strong vibration, the most distant. As with VAWTs, the operation of this type of turbine can be improved with the use of diffusers, but one of the most prosperous ideas is to equip the rotors with systems designed to adjust the rotor blade arrangement according to wind parameters ^[74][77]. Changing the angle of attack is also important for safety reasons, such as in situations where the turbine must be stopped. Both groups of wind turbines (VAWTs and HAWTs) can be installed on land or water. The second way may allow them to work better due to the absence of any obstacles and high wind speeds. For this purpose, turbines with a horizontal axis of rotation are best suited, as they can withstand higher overloads. There are studies ^[75][78] testifying to the possibility of creating floating wind farms. This would allow the use of waters too deep to place a foundation and could also minimize costs. For all types of turbines, it is very important to choose the right material to create their blades. It must be light enough, but also strong enough to achieve the best performance. Most often composite materials (carbon and glass fibers) are used. However, this is one of the biggest problems for wind turbines because these materials are difficult to recycle. Initial research shows the possibility of using used rotor blades to make composite plates, which can be used to make structural components for bridges or buildings. Researchers ^[76][79] have shown that proper processing allows even higher strength than analogous plates created for this purpose.

2.4. Water Turbines

The use of the potential or kinetic energy of water to produce energy is the main RES worldwide ^[33][36]. From the point of view of technologies, the types of hydropower plants adopted are dammed ^[77][80] and pumped storage ^[78][81]. The first type involves restricting the flow of a river by damming the water, creating an artificial basin, and at the same time increasing the possibility of utilizing the high head to produce electricity. For this purpose, Francis ^[79][82] or Pelton ^[80][83] water turbines are used, depending on the available head of the water. The second type uses mainly the potential energy of water. Hydropower plants may be used as a type of energy storage. In fact, when there is a peak of power production on the electrical grid coupled with a relatively low price of electrical energy, i.e., excess energy is produced and the energy is cheap, water from the lower reservoir can be pumped into the upper one, while when there is a peak of demand of energy in the grid and the price is relatively high, the water can be released into the lower reservoir, giving back the energy stored. These types of turbines are rarely used for small hydropower plants ^[81][84]. For this purpose, a large proportion of other means of energy production, consisting of flow or tidal power plants, can be used. The first of these are located in rivers, where they use the kinetic energy of water to produce mechanical energy and then electricity. Historically, river mill wheels were used for these purposes. As of today, the best universal system cannot be directly selected. Depending on the case, it could be, for example, an Archimedes screw ^[82][85] or a Kaplan turbine ^[83][86]. Properly fitting a small hydropower plant into a given landscape is a very difficult task. One should keep in mind the relatively negative impact of such an installation on the environment. The construction of such a power plant can cause problems in fish migration, disturb the biological balance of the river or pollute the water itself. However, studies indicate that if a number of guidelines are followed and particular attention is paid to the development of the plant, these problems can be minimized ^[84][85][87,88].

2.5. Biomass Technologies

In the era of transitioning away from conventional energy sources, there is an increased search for zero-carbon alternatives. One possible method of clean energy generation is the combustion of biomass. Biomass can be defined as any organic substance of biological origin (vegetable or animal) available in the world. When using biomass processing technology, the most chosen biomasses are municipal waste, agricultural waste, vegetables, and energy crops ^[86][89], as well as wood in unprocessed form or pellet form. In addition, these biomass types can be converted into biogas or biofuels, which can have much better energy conversion parameters compared to the raw/origin material ^[87][90]. In addition to the type of fuel, different ways of burning biomass can be distinguished, depending on the scale of customer needs. For high-capacity units, fluidized beds ^[88][91] and grate boilers ^[89][92] are commonly used devices. The former has the highest efficiency, which derives from the characteristics of the combustion itself. Fuel is pulverized in the combustion chamber to form a slurry, with sand or ash particles adopted as inert material. For residential customers, the most popular way to obtain heat from biomass is by a fireplace. The installation can be adapted to transfer energy around the building, as well as to heat domestic water by burning wood in the fireplace ^[90][91][93,94]. Commercial biomass boilers are based on the use of wood or pellets, while in the case of using straw, batch boilers are often used. These boilers can be characterized by very high output (up to 1 MW), but there are also smaller units for a single household. Multistage biomass combustion shows promising results ^[92][95]. This consists of drying, pyrolysis, oxidation, and reduction stages. By converting solid fuel into a gaseous state, higher efficiencies are achieved throughout the process. Recent studies indicate that despite the inclusion of biomass as a renewable energy source, emphasis should be placed on the development of cogeneration technologies that allow obtaining not only heat but also electricity or cooling from combustion ^[93][96]. Biomass combustion is one of the most popular methods of heat generation in distributed renewable energy applications. Households without access to district heating or the natural gas infrastructure most often use units that burn wood or pellets. It is worth noting that suitably adapted systems can be used to generate electricity. The heat generated by a biomass combustion unit is transferred to a substance that changes the state of matter or, in general, to the working fluid of a power cycle. It then drives a steam or gas turbine, which, connected by a shaft to a generator, produces electricity. However, this way of biomass use can be more efficient when it is coupled with a combined heat and power (CHP) approach. In such a CHP system, components that take off excess heat allow one to supply thermal energy to the user. Such a solution works well in large central CHP plants, but also in distributed power generation ^[94][97].

2.6. Heat Pumps

Currently, there are relatively few methods of heating buildings that are not connected to a district heating network. Focusing on the use of renewable energy sources, in addition to the solar or biomass conversion systems already discussed, heat pumps are becoming increasingly popular. The use of these devices, particularly in the distributed energy sector, is considered a critical method for decarbonizing heat production worldwide ^[95][98]. Their operation is based on extracting energy from a low-temperature source to a higher-temperature source. The whole process must be assisted by supplying external energy, such as electricity. Heat transfer is carried out by means of a working medium consisting typically of an organic fluid, which must have appropriate operating thermodynamic parameters depending on the temperature of the lower source ^[96][99]. The most important of these is the boiling point. The medium is supposed to receive heat in the evaporator, from a medium with a relatively low temperature. The lower heat source can be a variety of different media. Often, it is the air surrounding the building ^[97][100]. Pump systems developed for this purpose have fans that allow heat transfer from the air to the working medium. The great popularity of this solution is directly due to its relative versatility. An air heat pump can be installed on a building in a variety of climatic conditions. The most stable operation of these devices is observed for ambient temperatures in the range of −3 °C–10 °C ^[98][101]; however, after appropriate adjustment of the operation of the units, optimal operation is observed even at temperatures down to −30 °C ^[99][102]. The lower source of the heat pump can also be the ground. Direct energy extraction from globally available deposits is often uneconomical and requires specific conditions ^[100][103]. Most often, small geothermal deposits have too low a temperature to heat domestic water and supply heating directly or generate electricity. This energy stored in the ground can easily be used in heat pump systems. The main advantage of these solutions is the relatively stable temperature of the lower source, which translates into better heating efficiency. Ground-source heat pump applications can be divided into two types: vertical or horizontal heat exchangers. The first ^[101][104] are based on drilling a borehole reaching typically from 30 to 100 m deep or more. This procedure is designed to ensure a lower heat source with the highest possible constant temperature. With depth, the temperature of the ground tends to be more constant over the year; however, at the same time the investment costs associated not only with digging the borehole itself but also with the circulating pump with the required head increase. As regards the horizontal ground heat exchangers, they involve laying a ground collector with a large area at a depth of 50–120 cm ^[102][105], making them more susceptible to ground temperature variation at shallow depths. In addition to extracting energy from the ground, heat pumps can also extract thermal energy from surface water ^[103][106] or groundwater ^[104][107] on a similar basis to ground pumps. Also worth mentioning are the possibilities of integrating heat pumps with solar ^[105][108] or photovoltaic ^[106][109] systems. In the first case, solar collectors can act as a lower heat source, reducing the temperature difference between the evaporator and condenser, and increasing the efficiency ratio of the system. A photovoltaic system very often goes hand in hand with heat pumps, since the electricity produced by PV can easily be used to power a circulating pump. The use of electric compressor heat pumps is not the only way to transfer heat energy from a medium with a lower temperature to a higher one. It is also possible to distinguish systems that base their functionality on the process of absorption ^[107][110] or adsorption ^[108][111]. Unlike compressor pumps, these require the application of energy in the form of heat to the system. The main part of the absorption-based system is the circulation of usually a solution of water and lithium bromide (LiBr) salt. It is also possible to use other substances with similar sorption characteristics. During the operation of the system, when heat is supplied to the mixture, the absorbent material separates from the water and becomes the energy carrier. After giving up the heat, it reunites with water, and the process repeats itself. If it comes to adsorption processes, a mixture of two substances is not observed in it. Water in this process is deposited on the adsorbent material, often silica gel ^[109][112]. In this activity, the system gives up heat, and conversely, in the case of heating the sorbent, water is released. This type of heat pump requires an external heat source, most often a gas burner or hot water in traditional applications or thermal energy from renewables. For the latter, it is possible to use a number of heat sources, such as solar collectors ^[110][113] or a biomass burning unit ^[111][114]. The described heat pump systems are assumed to have the process of heating domestic water or indoor air. However, it is possible to change the operation mode of the device from heating to cooling, shifting the role of the condenser to one of an evaporator, and vice versa, by means of a valve system. This allows you to receive energy from the utility. Apart from reversible vapor compressor heat pumps, a common application consists of the use of the previously mentioned sorption heat pumps as a chiller. This is a very common approach in hybrid renewable polygeneration systems in order to produce cooling through the use of heat. In order to give a good idea of the approaches taken by the authors of the various works to improve the performance of the cited technologies, Table 1 was created. It describes how the researchers dealt with the problems of the various RES technologies and shows what results they obtained.

Table 1.

Summary of methods on improving the performance of the listed technologies.

Technology	Problem	Solution	Result	Reference
PV	PV module surface temperature too high	Integration with a system that uses excess heat	Decreased cell temperature, increased cell efficiency	[36][39]
		Integration with a system spraying surface with water	Decreased cell temperature, increased cell efficiency	[37][40]
	PV module efficiency or annual irradiation too low	Focusing solar irradiation with lens or mirrors	Increased efficiency, very high surface temperature	[42][45]
	High influence of shading on efficiency of PV module	New approaches in MPPT optimization	Lowered sensitivity of PV modules to changing external conditions	[45][48]
Solar thermal	Possible higher efficiency of whole system
[
99
]
[
102
]

Table 1 shows that there are many ways to improve the operation of RES technologies. However, it is worth mentioning that not all the methods shown above carry only positive aspects. An example is the idea of concentrated PV cells [21]. This idea, despite the theoretical increase in efficiency, also translates into a significant increase in the surface temperature of the cell, which results in a decrease in efficiency. Similarly, the idea of creating hybrid PV/T [26] modules can be problematic. The issue with this solution is the requirement of specific weather conditions to take full advantage of this system, which directly translates into its cost-effectiveness. An analogy can be made with the creation of diffusers for wind turbines ^[38][41], which can increase their efficiency, but depending on the situation, may or may not be useful. The problem here may be the space utilized by this component. Instead of a diffuser, the turbine itself could have larger dimensions, which would translate into better performance. The ideas described above show that in most cases, there is not a universal way to improve the performance of RES technology, since several ways require specific conditions. This shows how much emphasis should be given to the proper selection of components of a classic or hybrid installation.

2.7. RES Technologies in Literature

Selected examples of systems based on renewable energy sources from the literature were collected and are summarized in Table 2. In order to denote the climate zone of the specified location, a symbol has been added next to the location of a given solution. The choice of symbols was guided by Köppen’s classification ^[112][115]. In the present contpapentsr, it was decided to use the first level of classification, which was described on the basis of average annual precipitation, average monthly precipitation, and average monthly temperature. The adopted classification includes the tropical humid (A), dry (B), mild midlatitude (C), severe midlatitude (D), polar (E), and highland (H) climate zones. It is important to note that this classification is also adopted for other tables in these contents paper.

Table 2.

Selected research on small-scale RES systems.

Technology	Power Installed	Energy Demand	Location	Energy Storage	Evaluation Method	Economic Review		Experiment	Reference
						Indicator	Value
PV	5.6 kW	5.7 MWh/year	Lublin, Poland (D)	No	PVSYST	ROI	8.9 y	No	[113][116]
	48 kW	383 MWh/year	Lublin, Poland (D)	No	PVSYST	ROI	5.7 y	No
	36 kW	-	Sverdlovsk, Russia (D)	No	PVSYST	-	-	No	[114][117]
	270 W	159.42 kWh/m2/year	Kumasi, Ghana (A)	Yes	TRNSYS	LCOE		Yes	[115][118
	Creation of hybrid PV/T module	Increased system efficiency, relatively low temperature of heated water	[	46	][49]
Possible higher collector efficiency	Focusing solar irradiation with lens or mirrors	Increased temperature and efficiency of collector	[49][52]
]
13.5 kW	93.8 kWh/month	Beijing, China (C)	Yes	Energy balance equations	LCR	0.95	No	[116]	VAWT	Negative torques of WT while operating	Integration with an augmentation system	Increased WT efficiency but waste of space which could be used for bigger rotor	[68][71]
Possible higher WT efficiency	Integration with wind accelerating construction (diffuser, building)	[62][65]
[	119	]	Relatively high needed starting wind velocity	Introduction of system controlling arrangement of blades	Increased WT efficiency	[74][77]
HAWT	Requiring large tracts of land, high sensitivity of wind conditions to land shapes, hard to place on deep waters	Building floating wind farms	Increased efficiency, space saving	[75][78]
WT	Hard to recycle components	Usage WT elements for other purposes	Positive impact on environment	[76][79]
WT	3 kW	3600 kWh (heat demand)	Edinburgh, Scotland (C)	Yes	DesignBuilder, TRNSYS	Comprehensive score	0.407–0.594	No	[117][120]
	5 kW	3600 kWh (heat demand)	Edinburgh, Scotland (C)	Yes	DesignBuilder, TRNSYS	Comprehensive score	0.473–0.638	No
	10 kW	3600 kWh (heat demand)	Edinburgh, Scotland (C)	Yes	DesignBuilder, TRNSYS	Comprehensive score	0.455–0.524	No
	1 kW	-	Forlì, Italy (C)	No	Mathematical analysis—Weibull	LCOE	0.61 €/kWh	Yes	[118][121]
	3 kW	2330 kWh	Ankara, Turkey (B)	No	MATLAB—Weibull	-	-	No	[119][122]
Hydrokinetic Turbines	5 kW	10,715 kWh	Baton Rouge, USA (C)	No	CFD	ROI	4–5 years	No	[120][123]	Water turbine	Difficult to implement as a small system	Possible places (near rivers) in-depth analysis, creation of guideline for such investments	Possibility of creating modern small hydropower plants in a greater number of locations	[84][87]
	Biomass
		5 kW	2620 kWh	Itacoatiara, Brazil (A)	No	CFD	ROI	6–7 years		No	Possible higher efficiency of the system	Introduction of multistage combustion
	3 kW	10,715 kWh	Increased efficiency of system	[	Baton Rouge, USA (C)92][95]
No	CFD	ROI		7–8 years	No	Introduction of CHP components	[94][
3 kW	97]
2620 kWh	Itacoatiara, Brazil (A)	No	CFD	ROI	15–16 years	No	Heat pumps	Possibility of lower source temperatures being too low	Introduction of different working media	Improved system reliability

When considering photovoltaic (PV) technologies, studies showing the operation of such systems in various locations around the world were collected. In this regard, it is a fairly universal technology. However, it should be noted that in locations characterized by low average irradiation, it will be less effective. Cieślak ^[113][116] pointed out that in the case of creating a basic system without an energy storage system, it is more cost-effective to install one that does not cover all the energy demand. In this case, an off-grid system would not be reasonable. The authors in ^[114][117] proposed a system with a solar tracer. They pointed out that such a solution is more efficient than a classic system facing in one direction. In addition, Agyekum et al. ^[115][118] analyzed the effect of high temperature on the energy loss of the system. Their paper was based on a comparison of the economics and efficiency of PV and PVT systems in Ghana. The results indicated that a stand-alone PV system had higher cost-effectiveness than a stand-alone hybrid system. It is worth mentioning that this situation becomes reversed when energy storage is added to the system. Li et al. ^[116][119] proposed an off-grid PV system with energy storage. The authors compared different system configurations and identified the most cost-effective approach. It consisted of 66% building demand energy storage and 1.4 energy penetration of the PV system. When analyzing stand-alone PV systems, the authors of publications are most often supported by software based on meteorological data and basic PV models (PVsyst) ^[113][114][116,117]. Such methodology allows one to analyze the performance of the systems depending on the geographic location and to analyze the efficiency of the system operation accounting for several design factors. In the case of a more complicated polygeneration system ^[115][118], one should turn to more complex software (such as TRNSYS). Most of the works available in the literature include an economic analysis. The most commonly used indicators are return on investment (ROI) ^[113][116] or levelized cost of electricity (LCOE) ^[115][118]. These approaches allow an assessment of the profitability of proposed solutions. Wind turbines (WTs) seemingly can perform well in a wider range of locations around the world. However, it should be noted that stand-alone wind turbines are not the most cost-effective solution in most cases. A suggestion from numerous studies is to create a hybrid system incorporating other systems in addition to the turbine. The authors of ^[117][120] indicated that a cost-effective approach would be to add an energy storage system to WT installation. The purpose of this system was to power a heat pump to cover the entire heat demand of the building. The authors undertook a comparison of such systems with different WT capacities and battery capacities at different locations in Scotland. Based on the comprehensive score method, they compared the investigated options. The results presented indicated that such systems could not qualify as the most profitable. However, the authors suggested that their use in Scotland would be possible with the assumption of subsidies from the government. The authors of ^[118][121] came to interesting conclusions. The study was based on a comparison between the actual production of a wind turbine and an estimated value based on weather data. The study showed the difference between the two situations and allowed for their economic analysis. The authors noted that the studied installation was not characterized by high profitability, and they concluded the analysis under the assumption of better wind conditions allowed satisfactory results. In general, the work showed that stand-alone wind systems are usually not economically efficient and that they could play a greater role in hybrid systems. Location is very important for wind power plants. Bilir et al. ^[119][122] undertook an analysis of the feasibility of an area in the vicinity of Ankara, Turkey for WT siting. The study indicated that the area does not have sufficient wind for large-capacity wind turbines, so the authors chose an alternative solution of three smaller WTs. The results showed that such an installation can produce enough energy to cover the electricity needs of small housing. It is worth noting that no energy storage system was assumed in this case, and thus most of the energy yield would be lost or transferred to the grid. Distributed hydropower is also worth mentioning. Puertas-Frías et al. ^[120][123] indicates that it is possible to use medium- and high-flow rivers for this purpose. Two locations were selected for analysis: Baton Rouge, USA on the Mississippi River, and Itacoatiara, Brazil on the Amazon River. The first step in the study was to select a suitable hydroturbine for the task of power production. A number of different profiles were considered to create a rotor with a horizontal axis of rotation. The selected parameters then made it possible to calculate their efficiency and also the cost-effectiveness of this solution. The systems studied were intended to be on-grid, and this fact showed that the economic viability of the investment is directly affected by a country’s policy regarding support for renewable energy sources. Without possible additional subsidies, the installation would not be profitable. ROI was used for the economic analysis, and the whole simulation was carried out in CFD software. It is worth noting that in the modern literature, it is difficult to find works based on the analysis of a small system entirely based on biomass. Common knowledge shows that while heating a household with biomass is profitable, in terms of electricity generation, it should go hand in hand with the production of other types of energy ^[121][124]. As devices characterized by relatively high electricity consumption, heat pumps should be used in tandem with systems that obtain energy from renewable sources. The use of grid energy in most cases is not among the most economical treatments ^[122][125].

2.8. Summary

Considering all the RES technologies mentioned above, it is hard to point out unconditionally the best one. Each has many advantages as well as disadvantages. However, it is possible to notice certain trends when it comes to the popularity of using a given system in scientific studies and also in practice. Biomass-based technologies can be considered the most popular. Although the data presented in Section 2.7 show that systems based exclusively on these technologies are no longer being developed to any great extent, it should be remembered that they are among one of the most popular sources of energy in the thermal sector in distributed and central ^[123][126] power generation. This is supported primarily by the fact that these technologies have existed for decades while the massive use of other RES is relatively new. These systems, in distributed power generation, are characterized by relatively low investment and operating costs. However, it should be noted that solar systems, which are becoming increasingly popular despite their relatively high investment costs, are a cheaper solution with a longer operating period ^[124][127]. It is worth noting that these days, there is a very strong emphasis on replacing fossil fuels. One of the solutions is the creation of biogas plants and biomass-based combined heat and power plants, thus covering the electricity and heat needs of small towns and villages ^[125][128]. The idea is to simultaneously produce energy and use the excess organic matter available in such agglomerations. Solar energy systems seem to take second place in terms of popularity. This is supported primarily by a certain versatility of this solution, the low complexity of basic installations, or the very large government support worldwide ^[126][129]. However, they have major drawbacks related to the stability of operation, which is also characteristic of wind energy-based technologies ^[127][130]. For this reason, the authors of several studies suggest the development of hybrid systems or the use of energy storage to improve the efficiency of the entire system when using this technology. Hydropower technologies account for a significant share of the RES mix in the world. However, these are large run-of-river, dam or pumped storage power plants. The use of water power in small systems is not a very difficult undertaking in terms of technology. However, it requires specific conditions, in particular the presence of flowing watercourses and the possibility to use natural or artificial basins. This fact translates into their lower accessibility and low popularity. Heat pumps are considered the future of the thermal sector. However, it is worth noting that for their operation they require the application of electricity, the price of which has been rising rapidly in recent years (2020–2022) ^[95][98]. This fact translates into an obligatory parallel use of renewable energy sources with heat pump systems in order for such an investment to be profitable.