**2. Energy Market**

The right energy structure and the increase in energy efficiency are the key issues on the way to the transformation in the field of energy, which is to ensure a safe future for all [8–11]. The current way of generating energy is associated with high greenhouse gas emissions because it is based mainly on fossil fuels and the inefficient use of traditional bioenergy, especially in developing countries [12–16]. Besides, the world's demand for energy is growing, which is largely due to the economic growth that drives global energy consumption. Rapid growth occurs in developing countries, such as China and India, while in countries belonging to the OECD (The Organisation for Economic Co-operation and Development), for example, Germany or Italy, energy consumption is beginning to stabilize but is still at a high level. These tendencies are in line with the assumptions of the Singer report [17], and this can also be confirmed by analyzing current trends, both in consumption and in energy production, as shown in Figure 2.

**Figure 2.** Total energy consumption and production, source: own elaboration based on data [18].

Let us first take a look at how global energy production has changed from a long-term perspective, both in terms of quantity and sources (Figure 3).

*Energies* **2019**, *12*, 3069

**Figure 3.** Global primary energy consumption 1800–2017 [19].

In the 1800s, almost all of the world's energy was produced by burning wood and other organic matter [19]. Oil consumption started in the 1870s [19]. Twenty years later, there were natural gas and hydroelectricity in the energy mix. Since 1900, coal consumption has increased significantly, and, now, it constitutes almost half of the world's energy [19]. In the 20th century, the energy mix changed significantly, coal took over traditional biofuels, and oil constituted up to around 20% of the energy market, and since 1960, the world has had nuclear electricity [18]. Today, on the energy market, we have renewable energy sources—RES, not appearing until the 1980–1990s [19].

Besides, apart from analyzing the total consumption of primary energy, we could also compare the contribution of different energy sources. As shown in Figure 4, changes in the energy mix are slow, so, mainly, fossil fuels dominate [19]. Except for the emergence of nuclear electricity, the energy mix was fairly stable for at least fifty years [18].

Changes in the structure of the energy mix began after 1997 [20,21] when the first EU regulations on energy from renewable sources appeared [22–25]. They were stated in the European Commission's White Paper Energy for the Future—Renewable Energy Sources RES (December 1997). In 2001, the European Parliament and the Council adopted Directive 2001/77/EC on the promotion of electricity production from RES on the internal market, which determined the share of RES in total electricity consumption by 2010 (replaced by Directive 28/2009/IN). A breakthrough in the development of RES took place in March 2006, after the publication of the Green Book [26].

**Figure 4.** Primary energy consumption by source, European Union [19].

Figure 5 shows primary energy consumption from 1965–2017 in continental regions [19].

**Figure 5.** Global energy consumption by region [19].

In 1965, most of the total energy was consumed in North America, Europe, and Eurasia. In total, they accounted for over 80% of global energy consumption. Consumption in other parts of the world has been increasing, most dramatically in the Asia Pacific. In 2015, Asia Pacific was by far the largest regional consumer. The total consumption (about 43%) was the same as in North America, Europe, and Eurasia combined [19]. The Middle East used 7%, Latin America 5%, and Africa 3% [18,19].

In 2017, the production of energy from coal increased due to China. Higher energy prices in the world had caused a drop in oil and gas production in the United States. Energy production in the European Union decreased due to moderate growth in energy consumption, lower primary energy production (nuclear and hydro), depletion of oil and gas resources, and climate policy, which ultimately means giving up coal. Large oil and gas exporting countries, such as Russia, Iran after international sanctions, Canada or Nigeria, as well as fast-developing countries (India, Indonesia, Turkey, and Brazil), were the main energy donors. In 2017, total consumption was at 567 EJ (EJ = exajoule = 10<sup>18</sup> joules), and it is forecast that by 2020, there would be a 10% increase [19].

Prices can have a big impact on the choice of energy sources. While comparing costs between sources, it is important to have relative prices [19]. In the literature, the levelized cost of electricity (LCOE) is used [27]. The concept of LCOE is defined in the following way:


Figure 6 shows the levelized cost of electricity (LCOE). The cost range is represented on the column chart for each technology. The white line in each of them represents the global cost of each technology. The average range of fossil fuel costs is shown as a gray column. In this chart, it is shown that, in 2016, most renewable technologies were in a competitive range to fossil fuels. The key exception was the thermal solar radiation, which remained about twice as expensive, but began to decrease [19]. Hydropower is the oldest renewable source. In this technology, the price is low. Analyzing how the average cost of technology changed in the years 2010–2016, we can see that the cost of solar photovoltaic system dropped significantly [19]. This reduction of cost in photovoltaic cells has been deep over the past few decades. The price of solar photovoltaic modules has decreased more than 100 times since 1976 [19].

**Figure 6.** Levelized cost of electricity (LCOE) in 2010 and 2016 [19].

Measuring the number of people with electricity access is a very important social and economic indicator. We think that the growth of population has caused an increase in energy consumption. Figure 7 shows that the percentage of people with access to electricity has increased at a global level [19].

**Figure 7.** Share of the population with access to electricity [19].

In 1990, about 73% of the population had access to electricity, and this level increased to 85% in 2014 [19]. High-income countries typically have access between 95–100% [19]. The growing global share was, therefore, driven by increased access in low- and middle-income economies. In many countries, this trend has developed, for example, access in India has increased from 45% to 80%, and in Indonesia, it has increased to 97% from 60% in 1990 [19]. While the trend is upward for most countries, there are some countries where this access is very low, for example, only 8.8% of Chad's population has electricity access. Therefore, we can assume that this factor will increase, and thus also energy consumption, in the future.

### **3. Materials and Methods**

The structures of the energy market and total energy production are important to determine the global heat emission. While producing useful energy, as it is shown in the introduction, heat energy is emitted (as an adverse side effect)—Equation (1). Moreover, sooner or later, almost 100% of that useful energy—electrical energy or chemical energy in fuels—is finally converted to heat energy. Because average efficiency of a thermal electric plant is about 40% [5–7], most of the primary energy (chemical energy in coal or natural gas, nuclear energy in nuclear fuel, etc.) is converted directly to heat. Engines in cars, where chemical energy in petrol is converted to mechanical energy, have similar efficiency. The results in Figure 2 show that the values of global energy production and energy consumption are very similar—they are approximately the same. Because the global annual useful energy production is known (Figures 2 and 3), taking the efficiency about 40%, total annual energy produced on the Earth could be written by Equations (1) and (2). Unfortunately, as it is shown in Figure 3, the amount of energy produced by renewable sources is still very low on a global scale. Therefore, it is reasonable to adopt the average efficiency of the energy production process at 40%, as in thermal power plants, because this dominates. The total power produced by our civilization on the Earth is determined by Equation (3).

$$E\_{\text{total}} = E\_{\text{heat}} + E\_{\text{useful}} \tag{1}$$

$$E\_{useful} \approx 0.4 \times E\_{\text{total}} \tag{2}$$

$$P\_{\text{total}} = \frac{E\_{\text{total}}}{3600 \times 24 \times 365} \tag{3}$$

where *Euseful*—annual useful energy production (electricity, motion, etc.)—it is given in Figures 2 and 3, *Etotal*—total annual energy, which is converted to heat (as an adverse side effect) and useful energy, *Eheat*—annual heat energy production, *Ptotal*—total power produced on the Earth (in Watts). All energies are in joules per year.

Based on the global energy production of useful energy (*Euseful*), it is possible to estimate the total energy (heat energy and useful energy), which is also finally converted to heat in consumers. At this point, the question arises whether the value of the calculated *Etotal* (in global terms) is high, low, significant, or insignificant. To answer this question, the total power produced on the Earth would be compared to the total solar irradiation reaching the Earth's surface. It is presented by factor n in Equation (4).

$$m = \frac{P\_{\text{total}}}{P\_{\text{GND}}} \tag{4}$$

where *n*—the ratio of total power produced on the Earth to the power of solar irradiation hitting the Earth's surface, *PGND*—solar irradiation power hitting the Earth's surface (the ground).

The heat produced on the Earth combines with the energy supplied from the Sun. Consequently, these two energies together cause the energetical effect as if the Earth was a little closer to the Sun. To illustrate this effect, we imagined the Earth before the industrial revolution (before the 19th century) in orbit with a radius of *<sup>r</sup>*0—Figure 8. Nowadays, additional heat emission causes an effect of the seeming shortening of the Earth's orbit to a radius of *r*1.

**Figure 8.** Global heat emission causes the seeming energetical effect as if the Earth was a little closer to the Sun.

The seeming shortening of the Earth's orbit is given by Equation (5).

$$s = r\_0 - r\_1 \tag{5}$$

Sun radiance, it means the radiant power per unit area emitted over all wavelengths and all directions (watts per square meter) of the Sun's surface area, is given by the Stefan-Boltzmann law [8]—Equation (6). The total power emitted by the Sun is given in Equation (7).

$$
\mathcal{R}^\* = \sigma \times T^4 \tag{6}
$$

$$P\_{\text{Sun}} = \mathbb{R}^\* \times A\_{\text{Sun}} \tag{7}$$

where: *R*\*—Sun radiance—power emitted by one square meter of the Sun's surface area (W/m2)


*T*—the temperature of the Sun's surface

σ—Stefan-Boltzmann constant 5.67·10−<sup>8</sup> (W m<sup>−</sup>2K−4)

The sunrays are emitted over all directions from the Sun. As we move away from the Sun (the distance from the Sun increases), the intensity of light decreases. For the distance "*r*" from the Sun, the solar radiation strikes the surface (sphere) whose area is equal to 4π*r*2. The surface power density (called also solar constant, flux density) for the distance "*r*" is represented by Equations (8)–(11).

$$P\_d(r) = \frac{P\_{Sun}}{4 \times \pi \times r^2} \tag{8}$$

$$P\_d(r) = \frac{A\_{Sun} \times \sigma \times T^4}{4 \times \pi \times r^2} \tag{9}$$

introducing the constant "*k*", we obtain:

$$k = \frac{A\_{\text{Sun}} \times \sigma \times T^4}{4 \times \pi} \tag{10}$$

$$P\_d(r) = k \times \frac{1}{r^2} \tag{11}$$

where *Pd*—power density, solar constant, power per unit of surface reached to place at a distance of "*r*" from the Sun in a unit of <sup>W</sup>/m2, *r*—distance from the center of the Sun (in meters)

Finally, the surface power density for the point at a distance of "*r*" from the Sun is inversely proportional to the square of the distance. Because the mean distance of the Earth from the Sun is about 149.6 × 10<sup>6</sup> km [9], the surface power density above the atmosphere of the Earth is about 1380 <sup>W</sup>/m2, theoretically based on Equation (9), and about 1366 <sup>W</sup>/m<sup>2</sup> in reality [10].

### **4. Results and Discussions**

Global useful energy produced on the Earth (Figure 2) in 2000 was equal 10,016 Mtoe (419.35 × 10<sup>18</sup> J), and in 2017 was equal 14,080 Mtoe (589.50 × 10<sup>18</sup> J). Total produced energy (heat + useful energy) is given in Equation (12) (transforming Equation (2)).

$$E\_{total} \approx \frac{E\_{useful}}{0.4} \tag{12}$$

The Sun always illuminates the surface of the Earth seen as a wheel. Total power of the solar rays reaching the top of the Earth's atmosphere (Equation (13)) can be computed by multiplying the surface power density (Equation (9)) and illuminated Earth's surface—the wheel with a radius of 6378 km [9]. Very rare phenomena, such as the solar eclipse, are omitted.

$$P\_{SE} = P\_d \times \pi \times R\_{Earth}^2 \approx 1.7 \times 10^{17} \text{ } \text{W} \tag{13}$$

where *REarth*—radius of the Earth, *PSE*—the power of the solar irradiation reaching the top of the Earth's atmosphere.

This result is confirmed by the Authors in work [11]. Around 8.1 × 10<sup>16</sup> W of solar radiation passes through the atmosphere and arrives at the surface of the Earth, which accounts for 47% of the solar energy that reaches the Earth [11]—Equation (14).

$$P\_{\rm GND} \approx 0.4 \mathcal{T} \times P\_{\rm SE} \tag{14}$$

where *PGND*—solar irradiation power hitting the Earth' surface (the ground).

The "*n*" ratio (total power produced on the Earth to the power of the solar irradiation ratio) is obtained in Table 1.


**Table 1.** Calculated values of *Ptotal* and "*n*" ratio. *PGND* = 8.1 × 10<sup>16</sup> W.

Taking Equations (11) and (14), (15) could be written as:

$$P\_{\rm GND}(r) = 0.47 \times k \times \pi \times R\_{\rm Earth}^2 \times \frac{1}{r^2} \tag{15}$$

where *PGND*(*r*)—solar irradiation power hitting the Earth's surface (the ground) as a function of the distance between the Earth and the Sun.

Next, we introduced the radius *r*0 of the actual (real) orbit of the Earth and the hypothetical radius *r*1 of the orbit, which is shorter and corresponds to a little higher solar power. It means the heat emission causes the e ffect of the seeming shortening of the Earth's orbit to a radius of *r*1. Because the *PGND* is inversely proportional to the square of the distance (Equation (16)), it is possible to determine the hypothetical radius *r*1 (Equations (17)–(19)). Finally, the seeming shortening of the radius of the Earth's orbit is given by Equation (20).

$$P\_{\rm GND} \sim \frac{1}{r^2} \tag{16}$$

$$P\_{\rm GND1} = P\_{\rm GND0} + P\_{\rm total} = P\_{\rm GND0} + n \times P\_{\rm GND0} \tag{17}$$

$$r\_1 = r\_0 \times \sqrt{\frac{P\_{GND1}}{P\_{GND1}}} \tag{18}$$

$$r\_1 = r\_0 \times \sqrt{\frac{P\_{\text{GND0}}}{P\_{\text{GND0}} \times (1+n)}} = \frac{r\_0}{\sqrt{n+1}}\tag{19}$$

$$s = r\_0 - \frac{r\_0}{\sqrt{n+1}}\tag{20}$$

where *<sup>r</sup>*0—radius of the actual (real) orbit of the Earth, *<sup>r</sup>*1—radius of a hypothetical orbit where the little higher solar power is the equivalence of the total energy produced on the Earth, *s*—seeming shortening of the radius of the Earth's orbit.

In conclusion, the heat emission as a consequence of energy production on the Earth gives the same e ffect as shortening the radius of the Earth's orbit by about 31,000 km in 2000 and about 43,300 km in 2017 with respect to the time directly before the industrial revolution (before the 19th century).
