1. Different Power Plants and Their Features

In power plants, primary energy sources like fossil fuels, biomass and other fuels are used to produce the electricity. Depending upon the electricity demand of a society, multiple number of electric generators and turbines are used in power plant. Then these generators transmit electricity to the grid and then through transmission lines this electricity is supplied to homes that is used to power our home appliances. Most power plants make use of steam or hydro turbines such as hydro power plant, geothermal power plants, solar thermal power plants and furnace oil based power plants [1]. Except solar PV power plants that do not use turbine rather they use solar PV modules and inverters to produce electricity.

Primary energy source for different power plants is different. Frequently being used primary energy source in power plants are nuclear fuels such Uranium, Coal, high furnace oil, natural gas and diesel. In hydro power plants, potential energy of stored water in dams at specific height is converted into kinetic energy through penstock and then this kinetic energy is used to run the turbine to produce electricity. Similar process happens in tidal power plants and ocean wave energy power plants. The working principle of wind power plants is quiet similar to that of hydro power plants. Only difference lies in utilization of kinetic energy of wind instead of water to run the turbine and produce electricity. Depending upon the location of country and availability of primary fuel, the power system of different countries is different. India use coal and biomass, Canada, Brazil and Switzerland use hydropower, US power system is based on natural gas and French power system is based on nuclear energy [2]. Visualization of different types of power plants have been depicted in following images:

2. Types of Power Plant

Power plants are categorized into two different types

1. Thermal power plants
2. Renewable energy power plants

2.1 Thermal Power plants

Thermal power plants use steam turbines and working cycle is mostly Rankine cycle. The basic working principle of these power plants is that they burn the fossil fuels to produce heat that is used to boil the water and convert it into steam. Then this steam strikes on blades of turbine and turbine in turn is coupled with generator through axil. Generator produce electricity which is further distributed to transmission system. The efficiency and life of thermal power plants depends upon the steam temperature and pressure. Thermal power plants are further divided into the following types.

2.1.1 Fossil fuel-based power plants

Current power system of most of countries is based on fossil fuels. In these power plants, chemical energy contained in fuel is converted into heat energy. Then heat is used to run the turbines to produce electricity. Not all of fossil fuel power plants work on steam turbines rather there are some exceptions. For instance natural gas power plants do not use steam instead their working principle seems to be similar with those of jet engines. Turbine is rotated due to pressure created by heat from burning natural gas. In case of combined cycle power plants, both the steam as well as pressure created due to heat is used to run the turbine. As per World Coal Association reports, the 37% of global electricity demand is still being met by using coal fired power plants [3].

Diesel plants are more space efficient as compared to coal fired power plants. Not only space efficiency but thermal efficiency also is higher for diesel power plants. But the maintenance cost and fluctuating prices of diesel have made diesel power plants less popular that’s why world power plants are mostly either hydro plants or steam based power plants. As compared to coal, oil and diesel, the price of natural gas is stable and its low carbon emission energy source. Therefore, countries based on coal fired power plants are shifting their power system on natural gas. As reported by number of natural gas based power plants has enhanced in 2019 by 3% and overall share of natural gas has seen a historical hike of overall 23% share in global electricity generation mix [4]. Then comes the combined cycle power plants are like natural gas power plants. The electricity generation from these power plants is more than any other standard power plant by using same fuel as in these power plants not only steam is used but also the waste heat is recovered and used for water preheating that enhances their overall efficiency. So the overall emissions from these plants for per kWh electricity generation are less than all other fossil fuel based power plants.

These fossil fuels based power plants are being discouraged since last decade due to their negative environmental impacts and hazardous emissions. Most of developed countries are shifting their power plants from fossil fuels to renewable energy power plants. 35 GW of renewable energy was added to US grid in 2020 and according to US president, US plans to shift its energy mix on 100% clean energy sources by 2035. To meet the decarbonizing goal of US by 20-30%, the RES capacity must be enhanced by 2-3 times [5]. Germany Plans to phase out non renewable energy sources in near future as it on cards that nuclear energy is going to be entire phased out by 2020 from Germany energy mix while the share of renewable energy sources (RES) will be enhanced. Coal is major contributor in electricity production of Germany and has share of 80% in electricity production. It is being planned to entirely phase out the role of coal in electricity production by 2038 [6] The electricity generation in UK has reduced from 395 TWh to 335 TWh after 2003 and this reduction was mainly due to reduction in coal fired power plant whose share has been reduced by 83%. Now natural gas as well as renewable energy sources are leading the energy market of UK [7]. These days only under developing countries are relying on fossil fuel based power plants or the remote areas are using diesel engines to produce electricity because they don’t have access to renewable energy technologies yet.

2.1.2 Nuclear Power Plants

It’s the form of energy when nuclei of two atoms fuse together into one nucleus or one nucleus split into two atoms. Fusion of two nuclei into one is called nuclear fusion while splitting of one nucleus into two nuclei is called fission reaction. Process of fission takes place in power plants and marine applications such as propulsion, where heavy atoms undergo fission process to release huge amount of energy due to chain reaction like in case of atomic bomb [8].

Figure 7: Nuclear Fission Reaction [8]

Back in 1956, the very first project of nuclear power was found commercially. This power plant named Calder Hall Power station was installed in UK. The basis for nuclear energy can be found in Einstein energy equation. According to that equation, mass of nucleus converts into energy as explained in following equation [9]:

Here E is energy released during mass conversion, m is mass of nucleus and c is speed of light. The energy from nuclear reactor is used to heat the water and produce steam. This steam is used to run the turbine which produces electricity. The fuel used in nuclear power generation is always less than fossil fuel used for same amount of electricity generation. To produce 1 GWe, 3 million tons of coal is required, while this amount of energy can be produced by using only 1 ton of nuclear fuel.

The nuclear energy has been widely used since long time for power generation. 20% of world electricity is produced from nuclear power plants. 8% of United States America energy comes from nuclear energy power plants. In Europe, nuclear energy seems to be major part of energy mix as in Europe, number of nuclear power plants is more than 200. The leading country in nuclear energy production is France whose electricity energy mix depends upon nuclear energy by 70%. By 2019, nuclear power plants had been installed in more than 31 countries. Out of these 31 countries, 14 countries were getting 20% of their annual electricity from nuclear energy. Global nuclear energy production is led by US, followed by France and china. Table 1 shows the nuclear energy capacity and its share in annual electricity generation, of top 5 leading countries in nuclear energy [10].

Table 1: Top five nuclear electricity generation countries, 2019

But future of nuclear energy seems to be unclear and confusing. Due to mishaps related to nuclear power plants destruction and the issues related to proper disposal of nuclear waste, world is observing decline in nuclear energy power plants. According to British Petroleum statistical report, a decline of annual 4% energy share has been observed in nuclear energy share. As nuclear energy was discouraged and decline in France, US and Japan by -0.4EJ, -0.2 EJ and -0.2 EJ respectively [11] [12] [13].

2.1.3 Geothermal Power plant

In geothermal power plants, there are three most famous types of power plants that are dry steam, flash steam and binary cycle power plants. The working principle of all of these power plants is similar to steam turbine power plants. Geothermal power plants are environmental friendly as compared to fossil fuel based power plants and have less hazardous impacts. Just like any other source of energy production geothermal energy also produce emissions of CO2, SOX and NOx, but this emission is less than conventional and other renewable resources. Here is the statistics showing geothermal emission as compare to other resources[14].

Figure 8: Emission Analysis of different renewable energies

Geothermal being the renewable and green energy source is most viable option to be used as energy source for domestic use and power generation because it is not only clean source of energy but also it does not have intermittency issues. In recent past, the capacity of geothermal power plants has been increased by 4% in 2013. By 2015, the installed capacity of geothermal power plants was 31.8 TWh per year [15]. There are many sites available in the world to extract geothermal resources [16]. According to the Geothermal Energy Association, it is expected that by 2021, global Geothermal industry will reach 1.8 GW[17] [18].

2.1.4 Solar Thermal Power Plants

Sun is huge source of energy. We use heat trapped in sun light for various activities such as drying of clothes fruits and get warm in winter. The light rays coming from sun rays can be concentrated at point by using solar concentrators which will increase the temperature of radiations. This rise in temperature can be even higher till 850 C. according to 2^nd thermodynamics law, all of this heat can not be converted into electricity and there will be some heat losses. There are many shapes sizing and different technologies of solar thermal power plants but they use basic material i.e. mirror in their construction to focus the sun light. The heat of concentrated solar radiations is used to convert the water into steam which is used to run the turbine and produce electricity [19]. The most famous and commercialized types of solar thermal power plants are known as parabolic troughs, solar power tower and dish engines.

The shape of parabolic trough concentrators (PTC) is like parabola and they focus the sunlight on line of parabola. To enhance the efficiency of PTC and keep it facing directly toward sun, single or double axis tracking may be used. PTC are capable to heat the water till 400 C.

In dish engine are called dish engines due to their shape which is similar to satellite dishes. The focus point of dish engines is at their focal length. Their efficiency is higher than PTC as they are capable to heat water till 750 C. A sterling engine then uses this steam and produce electricity.

In solar tower, multiple mirrors are used in field that concentrate the sunlight on a tower. These mirrors are called heliostat. The receiver at tower revives these concentrated radiations and converts the power of steam into electricity [20].

Figure 9 – Solar Thermal Power Plant

2.2 Renewable energy Power Plants

In renewable power plants, energy is directly harnessed from their respective flow source. Such as in hydro plant energy comes from water, in wind energy plant, energy comes from wind and in solar PV plant energy comes from solar energy. These power plants are sustainable in term of environmental impacts as well as energy sources. Only limitation associated with these power plants is intermittency of energy source such as solar and wind. That’s why these power plants are non-dispatch able as well as irregular [21].

2.2.1 Hydroelectric Power Plant

It’s a universal truth that water flows from higher potential to lower potential. Using this law, water is stored in dams at some height. Due to height, potential energy is stored in water. Then the water is made to flow from dams into penstock and potential gets converted into kinetic energy. When water hits the blades of turbine, the kinetic energy of water converts into mechanical energy. Now, turbine is coupled with generator, and mechanical energy of turbine is converted into electricity. When compared with other renewable power plants, the hydro power plants are dispatch able, reliable and have most matured technology. These power plants has zero emissions but they are threat to aquatic life as well as their capital cost is very high. Moreover, displacement is local population during dam construction and management of this displaced population is major issue [22].

2.2.2 Wind Power Plant

Wind carries kinetic energy which is converted into mechanical energy of turbine. The wind strikes the blades of wind turbine, the aero foil shape of turbine blade helps in rotation as aero foil shape increase the lift force and reduce the drag force. This not only increase the turbine torque but also increase the overall performance of turbine. Because if turbine rotates too slowly then most of wind energy pass through it uncaptured. That’s why the shape of turbine blade is kept in aero foil shape just like the helicopter rotor blade and airplane wings. Now the kinetic energy of wind has been converted into mechanical energy of wind turbine. The turbine is connected to gearbox through low speed shaft. The gearbox is used to enhance the number of rotations. Generator is connected to gearbox through high speed shaft and converts the mechanical energy into electrical energy [23].

On the basis of axis of rotation of the blades, it is divided into two parts. Two known basic types wind turbine are VAWT and HAWT. HAWTs are horizontal axis wind turbine while VAWTs are Vertical axis wind turbine. First wind mills that were being used in 900AD were VAWT. Origin of HAWT finds its foot marks in Europe where these wind mills were built to perform different mechanical tasks like water pumping and grain grinding. These ancient wind turbines had four number of blades along with yaw control mechanism mounted on a structure at some height. Fossil fuels based Industrial revolution resulted in popularity loss of windmills [24].Turbines size, design and power capacity continued to be developed in 20^th century leading to modern era wind turbines [24]. Advancement in VAWT continued in parallel with HAWT but they could not get much attention thus resulted in less financial support.

While 2020 was the best year in history for the global wind industry with 93 GW of new capacity installed (a 53 per cent year-on-year increase) a new report published by the Global Wind Energy Council (GWEC) warns that this growth is not sufficient to ensure the world achieves net zero by 2050. According to the scenarios that have been established by international energy bodies such as IRENA and the IEA, the world needs to be installing a minimum of 180 GW of new wind energy every year to limit global warming to well below 2°C above pre-industrial levels, and will need to install up to 280 GW annually to maintain a pathway compliant with meeting net zero by 2050 [25].

Wind is produced by natural process of uneven heating of earth surface thus it is renewable form of energy. Wind energy is unlimited, free, sustainable and localized source of energy. No SOx and NOx are produced during wind turbine operation. So, it is environmentally friendly and helps to reduce GHG emissions. Wind turbines have low O&M cost.

Wind turbine produce noise that’s why they are not recommended in urban areas rather in remote areas. The birds may strike with the blades of wind turbines and can be killed. The space efficiency of turbines is low. To avoid turbulence, they need to be installed at some specific distance with ground clearance. Wind energy is intermittent in nature and not available all the time thus it requires battery backup for constant energy supply [26].

2.2.3 Solar PV Power Plant

Photovoltaic systems, often known as PV systems, are power supply systems that use photovoltaic technology to convert useable solar energy into electric energy. It covers the arrangement of numerous components, such as solar panels, an inverter for receiving and converting sunlight into electricity, converting the solar terminal’s DC output power to an alternating current, as well as assembly, wiring, and other electrical components.

On the roof, ceiling panel systems capture and convert high-energy photons from natural sunshine, which is a useful kind of energy. The reliance on a community network to provide light, heat, cooling, and energy to your home can be decreased or eliminated by installing a high-quality solar system in your home. Installing an active solar system provides a clean, renewable energy source that requires no maintenance, saves money, and can be profitable in as little as a few years. You will enjoy many years of free energy once you have covered your solar system. The majority of solar power systems are network-based or grid-tied.

At the end of 2020, the global solar installed capacity was 714GW [27]. According to the International Renewable Energy Agency (IRENA), global solar power capacity will rise at a rate of 9% per year between 2018 and 2050, from 480 GW to more than 8,000 GW [18].

But intermittency of solar power plants will remain a key issue as during night, rainy days and cloudy days, the sun is not available and electricity production from solar power plants is zero. Just like wind turbines, solar panels also need ground clearance that reduce their space efficiency. Solar power plants are considered to be environmental friendly but the fact is that during their manufacturing, GHG emissions are not zero and also their recycling is major issue [28].

2.2.4 Tidal Power Plants

Just like other renewable energy sources, tidal energy is renewable source of energy that is similar to hydropower in its operation [29]. Tidal energy makes use of energy present in tides and convert it into mechanical energy and then into electrical energy. The first ever power plant based on tidal energy was Rance Tidal power station that started its operation back in 1966 [30].

The source of tidal energy is oceanic tides of earth. Tides are created due to gravitational forces of attraction exerted by other celestial bodies such as moon. The result of these attractive forces is creation of currents and motion in oceans. The short lived rise in level of water in oceans is observed when water level goes under a bulge due to strong attraction towards ocean. The shallow water is present adjacent to shoreline. When this short lived current volume meets with this shallow water then a tide is created. This is a natural phenomenon which continues to repeat itself because moon is continuously rotating in its orbit around the earth [31].

But tidal power plants are limited as their construction cost is high. Potential ideal sites are limited and found mostly near coastal areas. Mechanical stresses due to sea wave intensity can damage the tidal generation system. Moreover, these tidal power plants can cause destruction to natural habitat of aquatic life and result into migration of fish. Many obstruction like weak tides, frozen sea and straight shorelines reduces the feasibility of tidal power generation system. Technology is not mature, a lot of research and development is required. Transmission of power produced from tidal power plants to end user is tough and expensive [32].

Figure 10: Tidal Power Plant

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