Solar energy can be harnessed in two basic ways. First, solar thermal technologies utilize sunlight to heat water for domestic uses, warm building spaces, or heat fluids to drive electricity-generating turbines. Second, photovoltaics (PVs) are semiconductors that convert sunlight to electricity. Only 1.3% of U.S. electricity is generated with solar technologies, in part because direct costs are high.1
Solar Resource and Potential
- On average, 1.05 x 105 terawatts (TW) of solar radiation reach the Earth’s surface, while global electricity demand averages 2.5 TW.3,4
- Electricity demand peaks in the morning and evening, while PV generation peaks around mid-day. This is often referred to as the “duck curve” and leads to either energy surplus or deficits. Energy storage, and demand forecasting may play roles in eliminating these surpluses and deficeits.5
- PVs can be installed where electricity is used to reduce stress on electricity distribution networks, especially during peak demand.6
- PV conversion efficiency is the percentage of incident solar energy that a PV converts to electricity.7
- Though most commercial panels have efficiencies from 15% to 20%, some researchers have developed PV cells with efficiencies approaching 50%.8,9
- Assuming intermediate efficiency, PVs covering 0.6% of U.S. land area would generate enough electricity to meet national demand.7
- In 2011, the Department of Energy announced the SunShot Initiative. Its aim was to reduce the cost of solar energy by 75%, making it cost competitive with other energy options. In 2017, DOE announced that the 2020 goal of utility scale solar for $0.06/kWh had been achieved three years earlier than expected. The 2030 goal includes reducing utility scale solar energy to $0.03/kWh, allowing it to out compete traditional fossil fuel energy resources.10
Annual Average Solar Radiation2
PV Technology and Impacts
- PV cells are made from semiconductor materials that eject electrons when photons strike the surface, which produce an electrical current.14
- Most PV cells are small and rectangular, and produce a few watts of direct current (DC) electricity.15
- PV cells also include electrical conductors called contacts, which allow for the flow of electrons to the external load, and surface coatings to reduce light reflection.16
- A variety of semiconductor materials can be used for PVs, including silicon, copper indium diselenide (CIS), and cadmium telluride (CdTe).17 Although PV conversion efficiency is an important metric, cost efficiency—the cost per watt of power—is more important for most power applications. Some very cost efficient cells do not have high conversion efficiencies.
PV TECHNOLOGY TYPES AND EFFICIENCIES9,11
PV Cell Diagram12
PV Modules and Balance of System (BOS)
- PV modules typically comprise a rectangular grid of 60 to 72 cells, connected in several parallel circuits and laminated between a transparent front surface and a protective back surface. They usually have metal frames for strength and weigh 34 to 62 pounds.17
- A PV array is a group of modules, connected electrically and fastened to a rigid structure.18
- BOS components include any elements necessary in PV systems in addition to the actual PV panels, such as wires that connect modules in series, junction boxes to merge the circuits, mounting hardware, and power electronics that manage the PV array’s output.18
- An inverter is a power electronic device that converts electricity generated by PV systems from DC to alternating current (AC).18
- A charge controller is a power electronic device used to manage energy storage in batteries.18
- In contrast to a rack-mounted PV array, Building Integrated PV (BIPV) replaces building materials to improve PV aesthetics and costs.19
- Some PV arrays employ a solar tracker. This technology can increase energy output by as much as 100%.20
2.2 kW Residential BIPV System13
PV Installation, Manufacturing, and Cost
- In 2018, global PV power capacity grew by over 100 GW and reached 509.3 GW, it has increased by more than 30-fold since 2008.24
- The top installers in 2018 were China (44.4 GW), the U.S. (10.6 GW) and India(8.3 GW).24
- Though installed PV capacity in 2018 was only 4% higher than 2017, compared to double-digit growth in previous years, more solar capacity was installed than all fossil fuels and nuclear energy capacity combined. Even with this significant growth, solar power only accounts for 2.2% of global power generation.24
- The cost of solar electricity has dropped over 85% since 2008. Certain analyses have shown solar power prices in the range of 2 cents/kWh.24 In comparison, retail electricity averaged 10.58¢/kWh for all sectors and 12.89¢/kWh for residential consumers in 2018.1
- In 2018, global investments in solar power dropped significantly to $130.8 billion. This is likely partially due to declining capital costs of PV systems.26
- PV systems or components are manufactured in over 100 factories across 30 states.17
World Cumulative Installed PV Capacity21,22,23,24
Installed Price, Residential, Commercial, and Utility Scale PV Systems25
Energy Performance and Environmental Impacts
- Net energy ratio compares the life cycle energy output of a PV system to its life cycle primary energy input. One study shows that amorphous silicon PVs generate 3 to 6 times more energy than are required to produce them.27
- Recycling multi-crystalline cells can reduce manufacturing energy over 50%.28
- Although pollutants and toxic substances are emitted during PV manufacturing, life cycle emissions are low. For example, the life cycle emissions of thin-film CdTe are roughly 14 g CO2e per kWh delivered, far below electricity sources such as coal (1,001 g CO2e/kWh).29,30
- PVs can reduce environmental impacts associated with fossil fuel electricity generation; for example, thermoelectric plants use an average of 15 gallons of water to produce one kWh of electricity.31
Solutions and Sustainable Actions
Policies Promoting Renewables
- The price consumers pay for electricity does not cover externalities such as the cost of health effects from air pollution, environmental damage from resource extraction, or long-term nuclear waste storage.32 For instance, in 2011, Harvard Medical School estimated the external costs of coal to be around $345 billion annually.32 Policies that support PVs can address these externalities to make PV energy more cost-competitive.32
- Proposed carbon cap-and-trade policies would work in favor of PVs by increasing the cost of fossil fuel energy generation.34
PV policy incentives include renewable portfolio standards (RPS), feed-in tariffs (FIT), capacity rebates, and net metering.35
- An RPS requires electricity providers to obtain a minimum fraction of their energy from renewable resources by a certain date.
- An FIT sets a minimum per kWh price that retail electricity providers must pay renewable electricity generators.
- Capacity rebates are one-time, up-front payments for building renewable energy projects, based on installed capacity (in watts).
- With net metering, PV owners get credit from the utility (up to their annual energy use) if their system supplies power to the grid.
What You Can Do
- “Green pricing” allows customers to pay a premium for electricity that supports investment in renewable technologies. At least 850 utilities in nearly all 50 states offer some version of this green pricing. Renewable Energy Certificates (RECs) can be purchased in addition to commodity electricity to “offset” electricity usage and help renewable energy become more competitive.36,37
- Two emerging PV technologies are bifiacial PV modules and concentrator PV (CPV) technology. Bifacial modules are able to collect light on both sides of the PV cells, which can improve electricity generation depending on environmental conditions. CPV utilizes low-cost optics to concentrate light onto a small solar cell. By reducing the area of PV cell needed, more resources can be focused on high efficiency cells.38,39
A watt is a unit of power, or a rate of energy flow. 1 TW = 1,000 GW = 1,000,000 MW = 1,000,000,000 kW.
A kilowatt-hour is a unit of energy. 1 kWh is the electricity energy required to light a 100 watt light bulb for 10 hours.
- U.S. Department of Energy (DOE), Energy Information Administration (EIA) (2019) Monthly Energy Review June 2019.
- U.S. DOE, National Renewable Energy Lab (NREL) (2012) “Photovoltaic Solar Resource of the United States.”
- Goswami, Y. (2007) Energy: The Burning Issue. Refocus, 8(3):22-25.
- U.S. EIA (2019) International Energy Statistics.
- U.S. DOE, Energy Efficiency and Renewable Energy (2017) “Confronting the Duck Curve: How to Address Over-Generation of Solar Energy.”
- America’s Energy Future Panel on Electricity from Renewable Resources, National Research Council (2010) Electricity from Renewable Resources: Status, Prospects, and Impediments.
- National Renewable Energy Laboratory (NREL) (2012) SunShot Vision Study.
- Energy Sage (2019) “What are the most efficient solar panels on the market? Solar panel efficiency explained.”
- NREL (2019) “Best Research-Cell Efficiencies.”
- U.S. DOE (2019) The SunShot Initiative.
- NREL (2018) Champion Module Efficiencies.
- Adapted from NASA Science (2002) “How Do Photovoltaics Work?”
- Photo courtesy of National Renewable Energy Laboratory, NREL-15610.
- U.S. DOE, Energy Efficiency and Renewable Energy (EERE) (2013) “Energy Basics: Photovoltaic Cells.”
- U.S. DOE, EERE (2013) “Energy Basics: Photovoltaic Systems.”
- U.S. DOE, EERE (2013) “Energy Basics: Photovoltaic Electrical Contacts and Cell Coatings.”
- Platzer, M. (2015) U.S. Solar Photovoltaic Manufacturing: Industry Trends, Global Competition, Federal Support. Congressional Research Service.
- U.S. DOE, EERE (2012) “Energy Basics: Flat-Plate Photovoltaic Balance of System.”
- Barbose, G., et al (2013) Tracking the Sun VI: An Historical Summary of the Installed Price of Photovoltaics in the United States from 1998 to 2012. Lawrence Berkeley National Laboratory, LBNL-6350E.2013.2017.
- Mousazadeh, H., et al. (2009) “A review of principle and sun-tracking methods for maximizing solar systems output.” Renewable and Sustainable Energy Reviews, 13:1800-1818.
- European Photovoltaic Industry Association (EPIA) (2014) Market Report 2013.
- International Energy Agency (IEA) (2016) Trends 2016 in Photovoltaic Applications Survey Report of Selected IEA Countries between 1992 and 2015.
- Solar Power Europe (2017) Global Market Outlook For Solar Power 2017-2021.
- Solar Power Europe (2019) Global Market Outlook For Solar Power 2019-2023.
- NREL (2018) U.S. Solar Photovoltaic System Cost Benchmark Q1 2018.
- Forbes (2019) “Clean Energy Accelerates As Global Investment Hits £332.1 Billion.”
- Pacca, S., et al. (2007) “Parameters affecting life cycle performance of PV technologies and systems.” Energy Policy, 35:3316–3326.
- Muller, A., et al. (2006) “Life cycle analysis of solar module recycling process.” Materials Research Society Symposium Proceedings, 895.
- Kim, H., et al (2012) “Life cycle greenhouse gas emissions of thin-film photovoltaic electricity generation.” Journal of Industrial Ecology, 16: S110-S121.
- Whitaker, M., et al. (2012) “Life cycle greenhouse gas emissions of coal-fired electricity generation.” Journal of Industrial Ecology, 16: S53-S72.
- Dieter, C., et al. (2018) “Estimated use of water in the United States in 2015.” U.S. Geological Survey Circular 1441.
- Fthenakis, V. (2012) “Sustainability metrics for extending thin-film photovoltaics to terawatt levels.” Materials Research Society Bulletin, 37(4):1-6.
- Harvard Medical School (2011) “Mining Coal Mounting Costs: The Life Cycle Consequences of Coal.”
- Bird, L., et al. (2008) “Implications of carbon cap-and-trade for U.S. voluntary renewable energy markets.” Energy Policy, 36(6): 2063-2073.
- U.S. DOE, EERE (2011) Solar Powering Your Community: A Guide for Local Governments.
- U.S. DOE, EERE (2013) “The Green Power Network: Buying Green Power.”
- U.S. DOE, EERE (2012) “The Green Power Network: Green Power Markets: Renewable Energy Certificates.”
- NREL (2016) Evaluation and Field Assessment of Bifacial Photovoltaic Module Power Rating Methodologies
- NREL (2017) Current Status of Concentrator Photovoltaic Technology