Abstract

Keywords

sunlight

energy

electricity demand

solar electricity

1.1. Introduction

This chapter begins with the connection between humans on Earth and the Sun that we depend on, and must adapt to, in order to survive. Then, we quantify how much energy is needed on average to sustain human life. This human energy demand is used as a benchmark when we next investigate electricity generation in the USA followed by global electricity generation. Solar electricity generation is compared with other electricity generation sources such as wind, natural gas, and coal.

1.2. Human-sunlight connection

The remarkable human eye contains photoreceptor cone cells that respond to the visible portion of the solar spectrum from ∼380 nm violet light to ∼740 nm red light with photopic (i.e., bright light) vision peak sensitivity corresponding to ∼555 nm green light [1], as shown in Figure 1.1. The near-ultraviolet light that is not absorbed by atmospheric ozone is used by our skin to synthesize vitamin D.

Meanwhile, oxygenic photosynthetic land plants (via chlorophyll and carotenoid molecules) effectively use visible light from the Sun to produce carbohydrate and perhaps the most benign of all waste products – pure oxygen that we breathe. The overall oxygenic photosynthesis process may be expressed by the chemical reaction given by H2O + CO2 → O2 + CH2O, where H2O is water, CO2 is carbon dioxide, O2 is oxygen, and CH2O is used here to represent a generic carbohydrate subunit.

Sunlight-dependent plants provide us plant-dependent humans with the food (carbohydrate) and oxygen that we need to survive and thrive. Plants provide us with clothing (e.g., cotton and linen), shelter for us (e.g., framing material from pine or thatched roofs from straw) and habitat for other animals, shade, furniture, cabinetry, decking and flooring, shipping crates, medicines and homeopathic remedies, paper and cardboard products such as books and boxes, dyes and pigments, perfumes and fragrances, cooking oils, delicious beverages such as tea and coffee, a sink for carbon dioxide, filtration of airborne pollutants, mitigation of soil erosion, coastal storm surge buffering, sound damping, wind breaks, sports fields, and simply enjoyment in our gardens, parks, and nature preserves.

Moreover, the Sun provides warmth, keeps in operation the hydrologic cycle (evaporation of water and precipitation), and in large measure dictates the Earth’s seasonal climate and weather that we must respond and adapt to. Humans even tailor their accessories to enable functionality in sunlight, for example, by using sunglasses and hats or by designing clothing for sunny days such as colorful women’s sundresses. With our natural connection to the Sun and solar radiant energy, it is inherently logical for humans to use sunlight for the purpose of generating clean electricity through the application of solar photovoltaic cells.

1.3. Human energy requirement

Each day, considering an average value, humans require ∼2.33 kWh (kilowatt–hours) of chemical energy to live a healthy life. Typically, instead of the units of kWh, this is expressed (in the USA) on food packaging labels as kcal (kilocalories) where 2.33 kWh is about equal to 2000 kcal [2]. Therefore, each month, on an average, humans require ∼70 kWh of energy. And, in one year, this equates to ∼850 kWh. Stated another way, powering a 100 W light bulb 24 h a day requires about the same energy as the human body each day. With ∼7.1 × 109 humans on the Earth in 2013 [3]; this is nearly equivalent to continually operating a quantity of 7.1 × 109 of the 100 W light bulbs, which results in a yearly energy expenditure of ∼6 × 1012 kWh or 6 PWh (petawatt–hours). Next, we will compare the demand for (chemical) energy that is required simply to power our bodies and sustain our lives to the demand for electricity that we then use to enrich our lives in technology-dense societies such as the USA.

1.4. Electricity generation in the USA

In 2013, total net generation of electricity (“all sectors”) in the USA was ∼4.06 × 1012 kWh [4]. At the end of 2013, the population of the USA was ∼3.17 × 108 [5]. If we normalize electricity consumption in the year 2013 by the population, we find that this leads to an equivalent of ∼1.3 × 104 kWh per year of electricity per person to power the USA and run its economy. This means that ∼35 kWh per day per person of electricity was utilized on an average, or ∼15× more energy per day than it takes just to sustain a human body in a state of good health (∼2.33 kWh per day) as we found in the preceding section. Therefore, on the one hand, while we need a sustainable agricultural base just to maintain human life by providing enough food (chemical energy), on the other hand, we see that living in a technology-dense society requires much more energy if we want to power industrial machinery and processes, run air conditioners and heat pumps, operate computers and charge mobile phone batteries, turn on the lights, and so forth.

Most of our electricity comes from thermal power plants that require either the combustion of fossil fuels or fission of radioactive materials. Specifically, the heat from combustion of coal or fission of uranium is used to convert water into steam; the steam is expanded in a steam turbine that in turn is connected to a generator that converts rotational mechanical energy into alternating current (AC) electricity. Natural gas may be combusted in a gas turbine that is coupled to a generator to generate AC electricity. In Chapter 4, we will discuss combined-cycle power plants that incorporate a gas turbine and a steam turbine that utilizes the waste heat from the gas turbine exhaust.

In 2013, electricity generated by the combination of coal, natural gas, and nuclear fission power plants yielded ∼86% of the total electricity net generation in the USA [4]. Therefore, this is where the motivation for solar photovoltaic cells comes from – the ability to generate electricity by using sunlight now, instead of the nearly complete reliance on fossil fuels such as coal and natural gas or fissile radioactive materials such as uranium (235U). As shown in Figure 1.2, solar photovoltaic electricity generation has recently been expanding rapidly in the USA. Nonetheless, in 2013, solar photovoltaic electricity net generation (“all sectors”) was ∼8.3 × 109 kWh, as shown in Figure 1.2, or only ∼0.2% of the total electricity net generation (all sectors) in the USA. This is actually much less even than the ∼7% per year waste of electrical energy (i.e., a staggering 2.79 × 1011 kWh in 2013) through losses associated with transmission and distribution [4].

There are a number of other ways to generate electricity including the use of wind turbines, hydroelectric plants, fuel cells, thermoelectric generators, and concentrated solar power – a solar thermal technology. As an example, in 2013, wind and hydroelectric, in particular, provided ∼4.1% and ∼6.6%, respectively, of the total electricity net generation in the USA [4].

For more clarity, solar photovoltaic electricity net generation in the USA is shown versus geothermal, wood biomass, wind, hydroelectric, nuclear (fission), natural gas, and coal electricity in Figure 1.3. Note here that the data in this section are from the U.S. Energy Information Administration’s Monthly Energy Review, 7.1 Electricity Overview and 7.2a Electricity Net Generation: Total (All Sectors) and Electric Power Monthly, Table 1.1.A. Net Generation from Renewable Sources: Total (All Sectors). Periodic updates occur and so there may be revisions to the data [4].

Before moving on, we will briefly discuss the combustion of fossil fuels for the purpose of generating electricity. Just in the USA in 2013 alone, there was ∼7.8 × 1011 kg of coal (anthracite, bituminous, subbituminous, lignite, waste coal, as well as coal synfuel) consumed...