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2.2 Renewable Energies

Site: Hamburg Open Online University
Course: Green Hydrogen
Book: 2.2 Renewable Energies
Printed by: Gast
Date: Thursday, 21 November 2024, 2:04 PM

Description

This chapter discusses renewable electricity as the central raw material for green hydrogen production. It highlights the three origins of renewable energy: geothermal heat, solar energy, and gravity. Technologies such as photovoltaic systems, wind turbines, and hydropower plants are used to generate renewable electricity. Hydropower and biomass have limitations, making photovoltaic systems and wind turbines the main options for large-scale green hydrogen production.

Table of contents

Overview

Electricity from renewable energy sources is, along with water, the central raw material for the production of green hydrogen. In addition, the most important technologies used for this purpose will be briefly presented.
As the figure below shows, all renewable energy flows or energy sources that can be converted into electrical energy come from one of three origins. These three basic origins are geothermal heat, solar energy and the gravity between the celestial bodies, basically between the moon, the earth and the sun. The sun's radiation energy is not only an energy stream itself but is also converted into various other renewable energy flows and energy carriers through a variety of natural processes inside the earth's atmosphere. Examples of this are wind energy and biomass. In case of biomass, the radiation energy of the sun is converted into a usable energy carrier by the photosynthesis of plant organisms. Wind is an energy stream that occurs due to spatial differences in the atmospheric air pressure distribution, which, in turn, result from variations in solar radiation.

Renewable energy sources, natural conversions and selected technical utilisation options
Renewable energy sources, natural conversions and selected technical utilisation options by Kaltschmitt et al. (2020), Fabian Carels (CC BY-SA)

 

These different renewable energy streams can be used to generate electrical energy. A variety of different technologies, which are shown in the blue boxes in the figure, are used for this purpose. On a global level, the following technologies are of particular importance for the generation of renewable electricity:

 
  • Photovoltaic systems
  • Solar thermal power plants
  • Wind energy turbines 
  • Hydropower plants
  • Biomass power plants
In some regions of the world, hydropower is already being used on a large scale to generate electricity. In Norway, for example, the share of hydropower in the total electricity mix is over 90 %. In Brazil, the electricity supply is also largely based on hydropower. Here, its share is about two thirds. However, the use of hydropower is often linked to considerable interventions in natural cycles. Rivers have to be dammed up, which on the one hand significantly changes their dynamics and on the other hand, can lead to the flooding of large areas. Additionally, in many regions, the usable potential for generating electricity from hydropower has already been largely exploited – for example in Germany. For these reasons, the use of electricity from hydropower is probably not an option for large-scale production of green hydrogen. The same is true for the use of biomass. As already explained, land-use conflicts, for example with food production, limit the extended use of biomass for energy production.
Therefore, the use of renewable electricity from photovoltaic systems, as well as wind turbines, will most likely be the main options for producing large quantities of green hydrogen. The basics of these technologies are briefly described in the following.

 

 

Photovoltaic systems

Solar cells are the basis for generating electrical energy in photovoltaic (PV) systems and are able to convert the radiation energy of the sun directly into electrical energy. The basic principle for this conversion is the so-called photoelectric effect. In the photoelectric effect, the energy of photons, which can be imagined as the smallest units of light, is transferred to electrons. To ensure that this energy transfer can take place and finally lead to the generation of electrical energy, the material of the solar cells is crucial.

Solar cells mainly consist of two thin layers of the element silicon. Silicon is a semiconductor and reacts to the photons of solar radiation by releasing negatively charged electrons from their positively charged counterparts. As now there is an electron missing, these counterparts are afterwards referred to as “holes”. Thus, these electrons now can move freely. By adding some boron to the lower silicon layers (“p-doped”) and some phosphorus to the upper layer (“n-doped”), an electric field is created in the solar cell – the semiconductor is “doped”. The doping ensures that the electric field is created in the boundary layer between the two silicon layers and the released electrons migrate to the n-doped layer. A conductive metal contact on the top of the solar cell collects the electrons. An electric current is generated. The electrons are guided via a metallic conductor to the consumer and further to the other side of the solar cell, where there is also a metal contact. In order to generate larger, usable amounts of electricity, individual solar cells are connected together to form modules (PV modules).
Structur of a solar cell
Structur of a solar cell by Name (CC BY-SA)

 

 

A central parameter for describing the technological performance of solar cells is the electrical efficiency. The electrical efficiency describes how much of the radiant energy arriving at the solar cell is converted into electrical energy. Currently, commercially available solar cells achieve an efficiency of 17 to 20 %. Most of the losses occur because only part of the incoming photon energy is transferred to the electrons due to physical limitations. Since solar cells generate direct current, but the electricity grid in Germany, as well as most consumers, need alternating current, PV systems are usually equipped with an inverter. The conversion in the inverter causes further losses, but these are small compared to the internal losses of the solar cell.

 

In order for PV systems to deliver maximum electricity yield, the modules must be oriented in such a way that solar energy irradiated onto the module surface is maximised. In the northern hemisphere of the earth, PV systems deliver maximum yields during the course of the day respectively the year if they are oriented towards the south. The optimal inclination of the modules depends strongly on the latitude - i.e. the distance to the equator - of the respective location. In Central European latitudes, the highest annual yields can usually be achieved with angles of inclination between 25 and 45°. If the focus is not on maximising the annual yield, but rather on providing electricity as constantly as possible throughout the entire year, a much steeper angle of inclination of about 60° is advantageous in Central Europe. The steeper angle of inclination leads to an optimisation of the (relatively low) electricity yield in winter, while the (higher) electricity yield in summer is reduced. Such a smoothing of the annual generation profile can for example be useful for private PV rooftop systems in order to maximise self-consumption of the generated electricity.
 
Stefan Müller: Status of

 

 

Wind energy turbines

 

Humans have been utilising the energy of the wind for over 2000 years. For the majority of this period, the focus was on converting wind energy into mechanical work, for example for irrigation purposes, milling grain or driving sailing ships. However, by the beginning of the 20th century at the latest, the use of wind energy was largely replaced by systems powered by steam, combustion engines or electric motors. In the last 40 years, however, the use of wind energy has experienced a renaissance in the course of the generation of renewable electricity. 

When talking about the use of wind energy, the wind capacity P_Wind is the key parameter:

                                                        P_Wind [W] = 0.5*ρ*A*v³
ρ is the density of air and a constant factor of 1.225 kg/m3. A describes the area through which the wind flows. The wind velocity v has a particularly large influence on the capacity of the wind. The capacity of the wind increases with the third power of the velocity, which means that a doubling of the velocity leads to an eightfold increase in the carried wind capacity. The average wind speed of a site therefore plays a key role in the search for suitable areas for the construction of wind energy plants.

Modern wind turbines slow down the wind and extract energy from the flowing air mass with the help of rotor blades. Wind turbines convert the kinetic energy of the wind first into mechanical energy (the rotation of rotor system) and then into electrical energy. However, it is physically impossible to extract all the energy from the wind: if we would extract all kinetic energy, the wind speed would become zero and then the path would be blocked by the non-moving air while the moving air would just go around the wind turbine so that no energy can be extracted any longer. Thus, a wind turbine can theoretically convert a maximum of 59.3 % of the wind power into mechanical power at the rotor. This ratio of extractable power to maximum wind capacity is described by the so-called Betz's coefficient.

The basic components of all wind turbines used today to generate electrical energy are:

  • Rotor blades
  • Rotor hub
  • Generator 
  • Power train with brake (and gear)
  • Wind sensors 
  • Yaw drive
  • Nacelle
  • Tower 
  • Foundation
  • Grid connection
Common, commercially available wind turbines almost invariably have three rotor blades. The rotor blades converge at the rotor hub. There, the rotational movement of the blades is transferred to the power train. The power train connects the rotor system (Rotor blades + rotor hub) with the generator, in which the mechanical energy is converted into electrical energy. Cables ensure the electricity transport to the base of the tower, where the wind turbine is connected to the grid. Another important component of modern wind turbines is the yaw drive, which enables the rotor to be aligned against the wind.
Structure of labelled wind turbine with visible interior of the nacelle
Labelled wind turbine with visible interior of the nacelle by Lisa Karies (CC BY-SA)

Two different wind turbine systems are currently in use: systems with gears and systems without gears. A gear transforms the rotary motion of the rotor system and the drive train to a higher speed. For wind turbines which have gears, low-cost, high-speed industrial generators can be used. Gearless wind turbines, on the other hand, work with expensive ring generators especially designed for wind turbines but save the expenses for a gear.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In addition to the limitation of the usable wind energy indicated by the Betz's coefficient, the energy conversion in the generator and, if present, the gear also lead to relevant losses. In the optimum case, currently available wind turbines can convert a maximum of around 45% of the wind capacity into usable electrical power.

The nominal power is an essential characteristic value for wind turbines and describes the electrical power output at the designated wind speed. In recent years, the nominal power of wind turbines available on the market has increased considerably. Meanwhile, turbines with a rated output of over 10 MW are being offered. However, turbines in the double-digit MW range are only installed offshore, i.e. at sea.

 
Development of wind turbines over time
Development of wind turbines over time. The year always refers to market availability or built prototypes by Fabian Carels (CC BY-SA)