Clean and renewable energy technology (CRET) consists of a variety of different energy sources and technologies that are environmentally friendly. These include photovoltaic (PV) and thermal solar, wind, geothermal, hydroelectric, Ocean Thermal Energy Conversion (OTEC), biofuel (ethanol being the most prevalent), and biomass. Geothermal energy can be harnessed through heat pumps, energy piles, and earth sheltering, while biomass can be utilized for combined heat and power (CHP).
In California, renewable energy sources generated roughly 32% of the state’s electricity in 2020, according to the California Energy Commission. To further promote the adoption of clean energy, the state has set a mandate to increase the share of renewable energy to 50% by 2030 through the Renewables Portfolio Standard (RPS) program. Renewable energy accounts for only 11% of the total energy consumption in the US as of 2020, up from 6% in 2006. Fossil fuels still dominate the US energy mix, accounting for 81% of total consumption.
Renewable energy sources offer a powerful means of reducing harmful pollutants such as CO2, CO, NOx, and VOCs from the atmosphere. Based on data from the Energy Information Administration (EIA), there was an overall reduction in CO2 emissions in the US between 2000 and 2020. The states of Ohio, Pennsylvania, and Indiana achieved the largest reductions in emissions, while North Dakota and Wyoming experienced an increase in emissions. This information updates the previous statistics from 2000-2009, which also demonstrated a decrease in CO2 emissions in the US, with Texas, New York, and Delaware leading the way. However, thirteen states, including Nebraska and Colorado, experienced an increase in emissions during this period.
The increasing number of electric vehicles (EVs) on the road has led to a surge in demand for charging stations, which can be powered by a variety of renewable energy sources such as solar and wind power. This can significantly reduce the environmental impact of EVs by reducing their dependence on fossil fuels. Although most charging infrastructure currently runs on grid electricity, a growing number of EV owners are utilizing rooftop solar power, with up to 42% of them charging their vehicles this way, according to a 2019 study by the National Renewable Energy Laboratory (NREL).
Advances in technology have led to the development of wireless induction charging for EVs, which transfers energy wirelessly from a charging pad (ground pad) to a receiving pad (on-board pad)to a receiving pad on the vehicle. Although this technology is still in the developmental stage, it holds the potential to make charging more convenient and efficient. Smart homes and Intelligent Transportation Systems (ITS) will also play a crucial role in the integration and management of charging infrastructure.
The two main types of wind turbines are horizontal-axis wind turbines (HAWT) and vertical-axis wind turbines (VAWT). HAWTs have a horizontal rotor shaft and blades that rotate around a horizontal axis, while VAWTs have a vertical rotor shaft and blades that rotate around a vertical axis. While wind power offers a promising future, wind turbines do come with a relatively expensive price tag of $1.2 million to $2.6 million per MW of installed capacity. To address this issue, a growing trend in the industry is the use of airborne wind turbines (AWT), which generate electricity and transfer it to a ground station via a power cable tether. However, it is important to note that wind turbines may face certain limitations due to the regulations of the Federal Aviation Administration (FAA), which aim to maintain clear and safe airspace for aircrafts.
Geothermal energy can be used for a variety of purposes, such as generating electricity or heating buildings. In 2021, there were geothermal power plants in seven states, which produced about 0.4% of total U.S. utility-scale electricity generation.5 In large-scale power plants, in order to harness geothermal energy, a well is drilled into the ground to access the hot water and steam that exists beneath the Earth’s surface. This water and steam can then be used to power turbines and generate electricity. There are three main types of geothermal energy systems: dry steam, flash steam, and binary cycle. Dry steam systems use steam that is directly produced from underground sources to power turbines. Flash steam systems use high-pressure hot water that is flashed into steam, which is then used to generate electricity. Binary cycle systems use lower temperature water to heat a secondary fluid, which then vaporizes and drives a turbine to produce electricity.
A three-stage process can be employed to efficiently utilize steam for power generation and heating purposes. The first stage involves using steam to drive a turbine or engine to generate electricity by heating water in a boiler until it turns into steam, which is then condensed back into water and returned to the boiler. The remaining heat from the steam is then used in the second stage for process heating, where it heats a process fluid like oil or gas, that is used in an industrial process. Finally, in the third stage, the leftover heat is used to heat domestic air or water via a heat exchanger.
For residential applications, geothermal energy is usually obtained through a ground-source heat pump (GSHP) system. A GSHP system uses the relatively constant temperature of the ground, usually several feet below the surface, to heat and cool a building. This is achieved by circulating a fluid, typically a mixture of water and antifreeze, through underground pipes, which absorbs heat from the ground in the winter and transfers heat to the ground in the summer. A GSHP system can also be used to provide hot water for a building. In this case, the fluid in the underground pipes is circulated through a heat exchanger, which heats the domestic water supply. A GSHP system has several advantages for residential use. It is a very efficient way to heat and cool a building, as it does not need to generate heat like a furnace or air conditioner. It is also a very reliable system, as the temperature of the ground is relatively constant year-round. Despite the initial cost of purchasing and installing a geothermal system, which can range from $7,000 to $30,000, with an estimated payback period of 5 to 10 years.
According to the U.S. Energy Information Administration, in 2020, hydroelectric power accounted for about 6.5% of the total utility-scale electricity generation in the United States, which is the largest percentage among renewable energy sources. Many dams are concentrated in the west where deep river valleys are present, with the Hoover Dam being a famous example located at the lower end of the Grand Canyon, and the Glen Canyon dam upstream. As of 2005, the US had 75,000 dams. Not all of these dams were hydroelectric power plants, and many dams were used for other purposes, such as flood control, irrigation, and recreation.
The Itapua Dam, located on the border between Brazil and Paraguay, produces a massive 12,600 MW of power, while the Grand Coulee Dam is the largest hydroelectric plant in the US, generating 6,809 MW of power from its three power plants, which is more than three times the power production of the Hoover Dam at 2,074 MW. In comparison, the Diablo One nuclear plant in California produces only 1,106 MW of power.
The erosion of the foundation of a dam is a common issue that can occur with any type of large structure. As the foundation erodes, it can cause leaks to occur, which can compromise the integrity of the structure and require costly repairs or construction work to prevent further damage. After 30 years, limestone erosion causes leaks to occur underneath the wall, necessitating costly construction to prevent further damage. In the 1970s, walls were extended from 80 to 150 feet to address this issue. Today, the required depth for wall extension has increased to 300 feet, but drilling to such depths can be challenging and expensive due to water pockets. To reduce drilling costs, the head of the wall is reduced to a third of its original capacity, which leads to a decrease in power output.
The sudden release of enormous amounts of water from the dam caused flooding in Nashville. The Teton Dam was built at a cost of $100 million but catastrophically failed in 1976. The US government paid $300 million in claims for damages, and the disaster resulted in the death of 11 people and 13,000 cattle. The total damage caused by the disaster was estimated at $2 billion, and the dam was not rebuilt. They conducted a $350,000 engineering study to assess the feasibility of rebuilding the Lake Delhi Dam which failed in 2010. As part of the pre-construction evaluation, it will be determined whether a moderate or high hazard dam is required. The former costs $10 million, with an additional $3 to $4 million required for electrical generation equipment. The latter increases the cost by $2 million. Reinforcing many dams is crucial, but determining the appropriate approach can be complex.
While dams only impound a small percentage (17%) of the total length of rivers, they can have significant impacts on downstream water flows, ecosystems, and communities. The El Atazar Dam, located near Madrid, Spain, was designed to supply water rather than electricity. However, a crack emerged in the middle of the arch dam due to foundation settling and concrete expansion.
Pumped storage hydropower systems can be utilized to enhance the dam’s functionality. This involves pumping water to a storage pool above the power plant during times of low electricity demand, thereby increasing the pressure head during periods of high demand. Furthermore, excess energy generated during low demand can be used to power the pump, a technique known as load balancing.
Ethanol fuel is a type of biofuel that is produced from the fermentation of plants such as corn, sugarcane, and switchgrass. The addition of ethanol to fuel has the potential to significantly reduce CO2 emissions and increase fuel supply (15%). E10 is a blend of 10% ethanol and 90% gasoline. It is the most commonly used type of ethanol fuel in the United States and is approved for use in all gasoline-powered vehicles. E85 is a blend of 85% ethanol and 15% gasoline. It is primarily used in flex-fuel vehicles, which can run on either gasoline or ethanol. E85 is mainly available in the Midwest and other parts of the United States where corn is abundant. Other types of ethanol fuel include E20, E30, and E50, but these are not as widely used as E10 and E85.
Cellulosic ethanol is produced from lignocellulosic biomass which refers to the fibrous plant material, including stems, leaves, stalks, and trunks, as well as a range of other plant materials such as grasses, wood chips, and agricultural waste (e.g., corn stover and wheat straw). It requires additional processing to break down the complex cellulosic materials into simple sugars using enzymes. These simple sugars can then be fermented to produce ethanol. The production of cellulosic ethanol is unlikely to have a significant impact on food supplies, as the feedstocks used for cellulosic ethanol production are typically non-food crops, crop residues, or other waste materials. Although the impact of using crops for conventional ethanol (i.e. corn ethanol) is a topic of debate, the actual land use required for this purpose is relatively small. According to the USDA, there are approximately 900 million acres of farmland in the US, with about 391 million acres used for crops in 2020.