What is renewable energy and what forms are there?

There are five common forms of renewable energy: hydropower, solar (photovoltaic and thermal), wind, biomass, and geothermal.  Among these, hydropower has been a major source of power for decades.  In the last 40 years, improved economics and performance has led to rapid growth in solar and wind power systems.

Within the next decade, electricity from renewable generation will be more competitive than coal, gas, and oil fired electricity generation.  Today, if you factor cost of healthcare and productivity losses that are a result of coal generated air pollution, renewables are already competitive.

How much energy does renewables provide today? What is the 2030 forecast?

In 2016, renewable energy provided 13% or 600 TWh of all electricity produced in the US in 2016.  This is in contrast to the fact that over 60% of power plants in the US and the world still use fossil fuels to generate energy.

Which renewables are growing the fastest in the US?

Wind and solar were the fastest growing renewable sources of energy in 2016.  Wind capacity at the end of 2016 reached 81 GW.  By the end of 2018, the US expects capacity to reach 95 GW.  Similarly, utility-scale solar generation will grow 44% from 21 GW at the end of 2016 to 31 GW at the end of 2018.  In spite of the high growth rates, the share of renewables will still be at 13% in 2018.

U.S. net electricity generation from select fuels (Source: EIA)

How much power do we use today in the US?

Under the American Clean Energy and Security Act of 2009, the total electricity demand in the US will increase from 4000 TWh/yr today to about 4300 TWh/yr by 2030.

What will be the share of renewables in the future?

The Department of Energy’s reference scenario (Annual Energy Outlook 2017) forecasts wind, solar, and natural gas to account for new capacity additions while coal generation will continue a steady decline in coming decades.  The contribution from renewables will increase to 1100 TWh or 25.5% in 2030 and 1350 TWh or 31.4% in 2040.

The increase in renewables will be dominated by wind and solar.  Wind generation will more than double between now and 2030.  Similarly, solar power will triple in this same time period.

How much electricity does renewables generate globally?

In 2015, renewable energy provided 5,660 TWh or 23.5% of the world’s 24,000 TWh electricity generation.  Among OECD countries, solar, wind, geothermal, and biomass increased 8.4% from the previous year.  Hydropower generation increased 2.2% from 2015.

By 2035, the IEA estimates that renewables will account for 30% of global energy.  A large part of the growth will be from solar PV, which is expected to provide 5% of global electricity consumption in 2030 and rising to 11% by 2050.

What are the challenges of integrating renewables onto the grid?

Supply and demand should balance during the operation of the grid.  However variability in generation caused when clouds are covering solar PV panels or when wind speed changes causes imbalance if the utility cannot address these fluctuations in time.  This is called intermittency.

There are daily and seasonal variations that limit renewable integration.  In California, the imbalance between peak demand and renewable energy production is greatest in the summer.  The problem results when net electricity demand rises steeply in the late afternoon when solar generation decreases.  This results in what is known as a “duck curve” profile for the net demand.

The duck curve shows steep ramping needs and overgeneration risk
(Source: CAISO)

In order to manage the sudden increase in demand for energy at sunset, expensive peaker plants are kept idle during the day and ramp up to supply the peak demand.  At the extreme, the California grid experiences a 14,000 MW surge in net load in the one hour period between 5 and 6PM.

Whether to manage the intermittency or the sudden rise in demand at sunset, next generation power companies are solving this by creating grid scale energy storage systems to shift and even out power supply.


How does hydropower work?

Hydropower generates electricity by harnessing the kinetic energy of water that flows downhill from higher elevations to lower elevations.  The water current spins a turbine to generate electricity.

How much hydropower is installed today?

Hydropower accounts for 6% of power produced in the US and 16% globally.  There are over 100 GW of hydropower installed in the US today.  The power generation capability of a hydropower turbine ranges from 100 kW to over 800 MW.

Most hydropower plants are located at large, federally built facilities.  For example, the hydroelectric generators at Hoover Dam produce 2 GW of electricity.  The largest hydroelectric dam is The Three Gorges Dam in China, is the worlds largest hydro electric dam generating a whopping 22 GW of power.

How does pumped hydro work?

Pumped hydro storage use electrically powered turbines to force water uphill at night to fill a reservoir.  The water is released to flow downhill to generate electricity during the day when demand is higher.

There are 40 pumped storage plants with a total capacity of more than 22 GW of storage capacity, roughly 2% of U.S. generating capacity.

Where are hydropower and pumped hydro found?

The state of Washington has the most hydropower generation capacity, accounting for 27% of hydroelectric generation in the country, followed by California, Oregon, New York, and Alabama.

Existing hydropower plants (Source: DOE)

In 2014 the United States was the 4th largest producer of hydroelectric power in the world after China, Canada and Brazil.


How does solar PV work?

Solar photovoltaic (PV) systems convert light from the sun into electricity.  Solar PV cells, made from semiconductor silicon, absorb sunlight (photons) at the atomic level and ejects electrons.  When electrical conductors are attached to the positive and negative sides of the cell, the electrons flow to complete a circuit, generating electricity.

Schematic of solar PV powered circuit (Source: ACS)

An individual PV cell, which ranges in size from 0.5 inches to 4 inches, produces between one and two watts of power.  The cells are connected in chains and sandwiched between protective glass and plastic to make a PV module.  To boost output, the multiple PV modules are connected together to make larger panels.

What is the energy efficiency of solar PV?

Commercially available PV cells convert light to electricity at 5% to 15% efficiency.  The current focus of research today is on developing new materials that increase the conversion of light to electricity.

How do solar PV systems connect to the grid?

Unlike other types of power generation, which produce AC electricity directly, solar PV produces direct current (DC) electricity.  In order to connect with the grid, PV panels use devices called inverters to convert the DC into AC.  The power is then aggregated and sent to a substation where it is integrated with the rest of the national electrical grid.

How large are solar PV systems?

Solar PV panels can be found in a wide range of sizes.  The solar PV systems found on homes, commercial buildings, and farms, also known as distributed solar systems, are on the kW scale.  Utility scale systems have at least 1 MW of generating capacity.

What is solar thermal and how does it work?

Solar thermal is another type of solar technology that converts sunlight into electricity. Instead of generating electricity from PV panels, it focuses light to generate heat.  The concentrated light produces heat.  These systems collect and concentrate sunlight onto small areas (thermal receivers) using lens and mirrors.  The heat is then uses generate steam to drive a turbine to produce electricity.

What is the energy efficiency of solar thermal systems?

Solar thermal systems that use dish shaped mirrors convert over 30% of the sunlight into heat.

How much electricity does solar produce?

Utility-scale solar now makes up about 2% of all utility-scale electric generating capacity and 0.9% of utility-scale generation.  This includes both PV and thermal technologies.  At the end of 2016, more than 21.5 gigawatts (GW) of utility-scale solar was in operation, of which 7.6 GW was installed in 2016 alone.

(Source: EIA)

Where are solar installations located?

Utility-scale solar installations are concentrated in California, North Carolina, Arizona, Nevada, and Georgia.  The largest solar PV plants in the US are in California.  The Solar Star PV power station located in Antelope Valley, CA produces 579 MW of electricity.

Utility-scale solar installations (Source: DOE)


How does wind power work?

Wind currents turn a windmill turbine around a rotor attached to a gearbox that transfers torque to a high speed, generating electricity.

Schematic of wind turbine (Source: DOE)

Who are the top producers of wind in the world?

The top five countries with the largest wind capacity are China, US, Germany, India, and Spain.

Where is wind found?

There are 74 GW of wind capacity deployed in the US.  The top five states for wind power are Texas, Iowa, Nebraska, Oklahoma, and California.

Existing wind farms (Source: DOE)

Globally, wind generation provided 4% of the world’s electricity in 2015.  This represents 433 GW of power generated.  Sixty-three GW of new wind power capacity was installed in 2015 alone.


How does biomass energy work?

Biomass energy comes from different sources including timber, manure, agriculture waste, and oil-rich algae.  Biomass is burned to heat buildings, provide industrial process heat, or to generate electricity.

In landfills, anaerobic bacteria, which live in oxygen-free environments, decompose organic waste to produce biogas, which is collected and burned to produce electricity.

(Source: EIA)

In 2015, 71 waste-to-electricity power plants used municipal solid waste (food, paper, cardboard, leather, yard, and wood waste) to provide 2.3 GW of power in the US.  These plants are concentrated primarily on the East coast.


How does geothermal energy work?

Geothermal power plants harness the heat from the earth to produce electricity.  The decay of radioactive elements in the earth’s core, produces geothermal heat (300°F to 700°F).  Geothermal heat can heat buildings directly but it also generates electricity by using hot water or steam from geothermal wells to power a turbine.

Existing geothermal plants (Source: DOE)