The Future of the US Power Grid System

Abstract

Renewable energy sources provide a great opportunity that will revolutionalized the US power grid system. Renewable energy accounted for 18% of the U.S power mix in the year 2017 and the adoption rate continues to rise. U.S public policies, technological innovations, and economics are the key drivers for change in the electrical power system. U.S power grid has advanced towards supplying reliable electricity from inexpensive and clean resources. However, renewable energy sources such as wind and solar present variable energy which causes reliability and power quality problems.  Thus integrating these sources to the power grid is much easier.  The U.S requires a high amount of transmission investments, additional reserve capacity and modification to power system operations to accommodate penetration of energy variable sources. On the other hand, geothermal and hydropower sources have the capability of generating electric power on a predictable and consistent basis.

Keywords: Power grid, wind, solar, geothermal, hydropower

Introduction

Energy drives the economy of U.S. In the year 2016, U.S spent $1.0 trillion of energy accounting to 5.6% of the GDP. The current energy sources that include fossils fuel, coal, petroleum and natural gas are associated adverse environmental impacts. The U.S has been adopting renewable and cleaner source of energy with advent of climate change policies [1].  Renewable energy deploys energies sources which are continually replenished by nature such as wind, sun, earth’s heat, and plants. Renewable energy technologies transform these energy technologies into electricity. Renewable energy increased to 18% of the U.S power mix in the year 2017  which is a rise of up from 15% in the previous year.   The shift was mainly driven by an increase in wind and solar driven projects. Wind and solar projects constituted about 62% of the new power construction in the year 2017 where more than 2.9% gigawatts of new renewable energy projects were initiated[1].  This paper analyzes the future of US power grid. The paper will compare the current power grid and the impact that renewable energies such as wind, solar and hydropower have resulted. The paper will determine if it will be possible to fully shift from the current power generating source to green sources. Current power Grid

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The US electrical grid comprises of a vast network of power plants, distribution centers, and transmission lines.  The generation stations essentially comprise of steam stations which deploy fossils fuels and hydro turbines which turns high inertia turbines leading to the generation of electricity.   Electrical power is sent across long distances by high-voltage transmission lines and local facilities or substations which steps-down the voltage and distribute it to various businesses and homes. “The U.S electrical grid comprises of 5,800 power plants, 3,200 utilities and power lines that covers over 2.7 million miles.  The U.S has three separate self-contained interconnections of power production and transmission which includes the Western, Eastern and Texas connections.”  The three grids are connected using high voltage direct current lines[2].    There has been a considerable technological advancement for the bulk power grid. The power grid system has been continuously updated with new technologies that include high voltage equipment and increased use of friendly energy generating sources.  There has been a rise in the use of power electronics in form of flexible alternating current transmission systems(FACTS) and power electronics in form of HV direct current(HVDC).

 The US power grid system has evolved over the years and has seen additions and retirements of various energy sources. Figure 1 below shows the evolving trend in electricity sources. 

Figure 1: US electric generation capacity additions and retirements between 2002-2016[3] 

Figure 1 shows a major shift in US power sources as the country moves into the future.   In the year 2016 more than 27 gigawatts of electricity generating capacity was added to the power grid system.  The additions offset the retirement of 12 gigawatts generation capacity.  Renewable energy sources especially solar and wind power have constituted the largest portion of additions. For example, during the year 2016, electrical energy additions to the power grid, more than 60% (8.7 gigawatts) originated from wind power and solar energy [3].

Future of Power U.S Grid

Impact of Solar power

U.S public policies, technological innovations, and economics are the key drivers for change in the electrical power system.  U.S power grid has evolved and the power system advanced towards supplying reliable electricity from inexpensive and clean resources.  Since the year 2010, more than 60,000 homes and 45,000 business across the U.S   are now relying on solar panels. In the year 2015, the number of solar power installations rose by more than 19% exceeding 25 gigawatts(GW). Solar power combined with biomass, wind and geothermal sources accounted for 7% of the U,.S power production[4]. The vision of the future grid is to provide a seamless, cost-effective system from productions to end-user with the capability of meeting all clean energy demands and capacity requirements.   The future grid system aims at scaling-up clean energy sources such as renewable sources such as solar and wind, natural gas and nuclear power.

The future grid will be characterized by advancement in computerized grid management systems for planning, real-time operations, and maintenance.  The introduction of environmental policies has prompted a shift in fuel mix from nuclear and coal generation to efficient natural –gas-fired combined units. Advancement in material science has led to new applications of power electronics such as a smart inverter for photovoltaic (PV) systems which interacts with the distribution system.  Innovations in the wind and solar energy generation and storage have resulted in both cost reduction and performance improvements. Technological advancement has reduced the cost of solar panels.  The future power grid system is expected to have a high penetration level of renewable energy sources and distributed energy resources (DERs). The abundance of natural gas in the United States has led to lead to increased use of natural gas to produce electricity.  The trend has been followed by a major reduction in nuclear and coal power generation which are viewed as less economical compared to natural gas[5]. 

 The development of natural gas-fired generators has triggered the retirement of less efficient coal and oil-fired generation, thus reducing carbon emissions.  On the other hand, hydro generators provide renewable energy to the power grid but continue to face increasingly stringent environmental restrictions which constrain the operations of traditional technologies.  Innovative hydro-designs which deploy variable speed drives offer flexible and improved efficiency in operations.  Additionally, hydro technologies that include marine and hydrokinetics types which captures tidal and current power of the oceans have continued to gain popularity due to their minimal environmental impacts. 

Impact of Windpower

“Wind power has become a major source of electrical power in the United States and help the country achieve its green power initiatives.  There are no technical limits or obstacles that prevent wind-generated power from being integrated into the national grid.  There are various factors has led to a growing trend in the generation of distributed renewable energy[6].

Wind power generation source has special characteristics including geographical distribution, economics, variability, and environmental benefits among others. Large-scale integration of both offshore and onshore wind power raises a number of challenges.”  Wind power may lead to grid infrastructural issues.  Optimization of the present infrastructure, reinforcements and extensions, offshore grids and better interconnections will lead to the improved capability of tapping the current wind power. Onshore wind technologies have been considered commercial but require further development to function to full-scale in harsh ocean environment.

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Future of wind energy

The energy department published a report in 2008 that had examined the technical feasibility of using the wind energy to generate 20% of the national energy demand by 2030. It indicated that wind energy is a viable renewable energy source in all the 50 states by 2050.  In order to attain the target, emphasis on transmission infrastructure was identified as significant. This is through streamlining the permitting regimes, improving reliability and the operability of the wind systems to increase the wind energy manufacturing capability. Increased turbine installation to increase energy collection was important. Therefore, turbine increase target was set at 7000 in 2017 from 2000 units in 2006. The integration of the 20% wind energy could lead to grid reliably for less than 0.5$ per kWh. However, it was identified it there is a challenge in setting and allocating cost for the new transmission line. Nevertheless, despite that challenge the availability of raw materials was determined to be a non-inhibiting factor[7].

The installation and the incorporation of wind energy to the grid have been significantly increasing over the years. At the end of 2016, approximately 82.1 GW of wind energy was installed and the figure is expected to rise. The improvement of technology and the application of subsidies are the key aspects that have made wind energy attractive energy source for developers. The figure below indicates the growth.

Figure 2: Wind Industry annual Market report [7].

Although the use of wind energy is continuously increasing, the turbine costs are not getting cheaper on cost per watt basis. In the early 21 century, the cost of turbines was cheaper than today. What has really changed is technology. The wind projects now have a higher capacity factor. This means that the watt of the wind has the ability to generate more electricity per year today than before. This is because the turbines now can spin faster than they use to in the past. Between 1998 and 2001, the capacity factor of a turbine was 25.4% but the projects build in 2015 has a capacity factor of 42.6%. Thus each watt installed lead to increased generation of 67% more electricity. Again, there are higher subsidies from the federal government for wind energy that for solar energy. wind energy has a production tax credit while solar gets an investment tax credit meaning it is cheap to generate wind energy production even without subsidized investment despite the reduction of subsidies for new projects by 20% from 2017.To increase the wind energy generation, offshore wind farms have become of interest to the government and private investors[8]. “For instance, the Block Island Wind Farm located 2.8 miles from the coast of Rhode Island become the first offshore wind plant in the US. Its five turbines have the capability to generate a total of 30 MW of power sufficient amount to allow the Blok Island Power Company to turn off diesel operating generators.” From 1st may 2017, the resident of the Rhode Island almost fully relay on offshore wind and the excess energy production was delivered to the mainland grid through an undersea cable. “NREL researchers have been analyzing the future of the offshore wind using the engineering and analysis expertise to help facilitate for an additional large-scale generation in the U.S coastal waters.” Based on NREL estimates, U.S offshore wind energy resource ranges from 10 million GWh to 14.4 million GWh annually.  Appendix 1 shows U.S offshore wind speed potential for a turbine of 80-meter height. U.S wind power resource estimates is highly dependent on various assumptions deployed to determine the power. This includes the height of the turbine and capacity factor since winds are stronger at greater heights above the ground.

Limitations of wind power

Although the wind industry has reached high levels of reliability, the unpredictable performance threatens the credibility of wind power and leading to increased financing costs. Maintenance and operation costs increase with the age of wind farms.   The power generated by wind turbines is uneven due to the unpredictable nature of wind.  The increase in a number of wind turbines in the power network makes their input difficult to manage for example regulation of frequency and voltage[9].

Solutions

The U.S continues to pursue taller towers and larger rotors in a bid to capture more wind power. It also continues research on improvement of rotors, blades, drivetrain components and controls. U.S renewable energy sectors have also increased research on strategies of enhancing the reliability of major components.  The path forward to reduce capital costs includes the reduction of mechanical and aerodynamic loads via advanced rotor and blade concepts. Reduction of turbine weights via the use of new and high strength materials and improved manufacturing process helps in reduction of capital costs.  Wind turbines require a  strong grid which imposes voltage and frequency to ensure they function properly.   The power grid must be capable of supply the necessary reactive power to the asynchronous generators to be able to absorb the power that wind turbines produce [10]. 

Hydro Power and Geothermal

Hydropower contributes a significant amount of energy in the US Power Grid system. Hydroelectric power is the largest source of renewable electricity in the U.S producing approximately 6.3% of the nation’s electricity.  US has approximately 102,867 MW of installed hydropower capacity with a hydropower generation of322,390 GWh as at 2017 [11]. The main goal of hydropower is to optimize and develops hydropower in a way that maximizes opportunities for low-carbon and low-cost renewable energy production.  Hydropower makes a major contribution in the nations clean energy generation initiative promoting environmentally sustainable growth. Hydropower has both societal and economic benefits which include the creation of employment, cost savings, reduction of air pollutants and greenhouse emissions. Geothermal energy is present in the entire country and marks the most highest-quality geothermal resources.  All states in the U.S may have the potential for electricity generation by deploying enhanced or engineered geothermal systems technology.  Geothermal electricity generation resources are categorized into three groups namely identified resources, undiscovered resources, and enhanced geothermal systems.   Identifies and undiscovered resources represent conventional geothermal resources whereby naturally occurring conditions such as permeability and high temperatures allows extraction of geothermal energy.  On the other hand, enhanced geothermal systems require engineering of rock permeability to creating geothermal energy extraction conditions [12].  The U.S generated geothermal power of 17GW in the year 218).   Appendix 2 shows the currently available geothermal sites with the favorability of deep enhanced geothermal system(EGS). 

Conclusion

The future US power grid would be a hybrid system of energy produced from several sources available.   Wind, solar, hydro and geothermal power are going to transform the U,.S power grid. Combination of various sources especially the renewable can maximize power generation in the grid. Addition of wind power to the grid system provides beneficial impacts such as reduction of emissions. However, the limited predictability and high variations in wind power lead to challenges in the implementation of this technology. 

Despite the benefits of the renewable power system, connecting renewable electricity generation plants to the electric power grid presents various technical challenges.  In particular, the increase in the use of energy variable sources such as wind and solar causes power quality and reliability problems. It may be difficult to balance between energy supply and demand.

Appendix 1

U.S Onshore Wind Energy Resource for 80-meter turbine height (Brown, 2011, p.13)

       Appendix 2: Geothermal power.

U.S Geothermal Resource (Brown, 2011, p.21).