Green Innovations Are Bringing
Energy-Saving Technology Home
Advances in technology and consumer demand for energy-saving devices have made green technology
increasingly accessible. Many innovations are geared toward homeowners looking to lower not only their energy bills, but also the carbon footprints of their homes and daily activities. From solar-harvesting shingles and windows to shoe insoles that can power a smartphone, this Yale Environment 360
gallery explores a few of these energy-saving technologies.
View the gallery.
29 Jul 2014:
Danish Wind Power To Be Half
The Price of Coal and Natural Gas by 2016
Wind power has overtaken all other energy sources as the cheapest form of electricity in Denmark, with a cost
roughly half that of coal and natural gas projected by 2016, according to
an analysis by the Danish Energy Association (DEA). Home to major turbine manufacturers Vestas and Siemens, the country has been investing steadily in wind power since the 1970s and seems to be reaping the benefits of those investments now, analysts say. Electricity from two new onshore wind power facilities set to begin operating in 2016 will cost around 5 euro cents per kilowatt-hour, according to DEA calculations. The Danish government aims to meet 50 percent of the country's total electricity needs with wind power by 2020, and another 20 percent with other renewable sources. By 2050, the government aims to produce all electricity from renewable sources.
22 Jul 2014:
Costs of Urban Light Pollution
Highlighted in Citizen Science Effort
A recently launched citizen science
project aims to highlight the environmental, social, and financial impacts of excessive nighttime lighting in cities around
the world. The project, called Cities at Night
, enlists people to help identify the cities pictured in thousands of blindingly lit photos taken by astronauts orbiting the earth. Organizers hope that when residents and officials see the bright photos of their cities at night, they will be prompted to cut nighttime light use and energy consumption. Widespread artificial lighting has made light pollution
a growing problem in urban areas by disrupting behavioral patterns of people and wildlife, wasting millions of dollars in energy costs, and adding greenhouse gases to the atmosphere. Some solutions are relatively inexpensive and straightforward, the organizers say, such as using shields to direct light down to street-level, which can allow a city to use lower-wattage streetlights.
21 Jul 2014:
India Doubles Coal Tax to
Fund Ambitious Clean Energy Initiatives
India's finance minister has doubled the tax on coal imported to or mined in the country, raising the tariff from $0.83 to $1.67 per metric ton, with plans to use the revenue to fund a host of renewable energy projects over the next decade, Clean Technica
reports. The revenue will be added to the National Clean Energy Fund, which was established to provide low-cost financing for renewable energy projects. The fund's scope will be expanded to include environmental projects as well as clean energy research and development, including a national wind energy program, four major solar power projects, and an initiative that aims to establish transmission corridors for distributing electricity from renewable energy sources. The revenue will also be used to fund a new, separate ministry focused on cleaning the heavily polluted Ganges River. The tax could raise as much as $1.2 billion in the first year, according to estimates.
18 Jul 2014:
Germany Tops Energy-Efficiency
Ranking and U.S. Scores Near Bottom
Germany tops a new energy efficiency ranking of the world’s major economies, followed by Italy, China, France, and Japan, according to
the American Council
for an Energy-Efficient Economy (ACEEE). The United States ranked 13th out of 16 nations, behind countries such as India, China, and Canada, although new carbon pollution standards proposed this June for existing power plants would be a major stride in the right direction, the ACEEE said. The group also admonished Australia, which ranked 10th, for demonstrating "a clear backward trend" in adopting energy efficiency measures. Germany took the top spot largely due to regulations it has imposed on commercial and residential buildings. And China, despite lax enforcement of building codes, uses less energy per square foot than any other country, the analysis found.
Five Questions for Jeffrey Sachs
On Decarbonizing the Economy
Thirty scientific institutions from 15 countries recently released a report for the United Nations outlining how
the world’s major carbon dioxide-emitting nations can slash those emissions by mid-century. Called the Deep Decarbonization Pathways Project
, the initiative aims to provide leaders with a plan of action in advance of a UN summit in September and climate negotiations in Paris in late 2015. Yale Environment 360
asked Jeffrey Sachs, director of Columbia University’s Earth Institute and a key player in the decarbonization project, five questions about the initiative and the prospects for global action on the climate front.
09 Jul 2014:
One-third of German Power
Came from Renewables in First Half of 2014
Thanks to abundant sunshine and wind, renewable energy generated 31 percent of Germany’s electricity
in the first six months of this year, according to a new report. The report
, released by the Fraunhofer Insititute, said that 27 percent of the country’s electricity production came from wind and solar, and four percent from hydropower. Solar power generation grew by 28 percent in the first half of 2014 compared to the first six months of 2013, and wind power grew by 19 percent over the same period. On a couple of particularly windy and sunny days in May and June, renewable energy accounted for 50 to 75 percent of Germany’s electricity production, the report said. The Fraunhofer Institute said that as Germany continues to phase out its nuclear power plants, it remains reliant on highly polluting “brown coal”
to produce electricity. A substantial portion of German coal-generated electricity is being exported, the report said.
02 Jul 2014:
Roughly $80 Billion Wasted on
Power for Networked Devices, Report Says
The world’s 14 billion online electronic devices, such as modems, printers, game consoles, and cable boxes, waste around $80 billion in electricity annually because of inefficient technology, according to a new report
the International Energy Agency (IEA). In 2013, networked devices consumed around 616 terawatt hours (TWh) of electricity, with most of that used in standby mode. Roughly 400 TWh — equivalent to the combined annual electricity consumption of the United Kingdom and Norway — was wasted because of inefficient technology. The problem will worsen by 2020, the agency projects, with an estimated $120 billion wasted as devices such as refrigerators, washing machines, and thermostats become networked. Much of the problem boils down to inefficient “network standby,” or maintaining a network connection while in standby mode. Most network-enabled devices draw as much power in this mode as when fully active, the report notes. Using today's best technology could cut energy consumption by 65 percent, the IEA said.
25 Jun 2014:
Blizzard Helps Scientists
Visualize Airflow Around Wind Turbines
A Minnesota blizzard has helped scientists understand airflow patterns around large wind turbines, paving the way for more efficient turbine designs and wind farm
Airflow patterns are visible during blizzard.
configurations, researchers report
in Nature Communications
. Wind farms lose roughly 10 to 20 percent of the potential energy they could harvest, and complex airflow patterns play the largest role in those energy losses. Studying airflow around large turbines, which can be more than 100 meters tall, is not feasible in lab settings, so scientists typically test smaller turbine models in wind tunnels and use tracer particles to visualize airflow patterns. Researchers from the University of Minnesota realized they could scale up their experiments to real-world conditions by using heavy snowfall during a blizzard to trace airflow patterns, as shown in this video. Their findings show that airflow patterns under real-world conditions differ from smaller-scale laboratory tests in important ways, and those differences should be taken into account when designing turbines and wind farms.
24 Jun 2014:
Concentrated Solar Power
Could Compete with Natural Gas, Study Says
Concentrated solar power (CSP) could meet a substantial percentage of current energy demand in some parts of the world, according to research
CSP plant in San Bernardino County, CA
published in the journal Nature Climate Change
. In the Mediterranean region, for example, the study shows that a grid-connected CSP network could provide 70 to 80 percent of current electricity demand, at no extra cost compared to natural gas-fired power plants. CSP could also feasibly meet energy demands in parts of southern Africa, according to researchers. CSP systems use mirrors or lenses to concentrate solar rays into a small area. The concentrated energy heats a liquid that produces steam to drive turbines, which means that the collected energy can be stored as heat and converted to electricity when needed — a major advantage over solar panels, which store energy much less efficiently.
19 Jun 2014:
Rerouting Flights to Avoid
Contrails Would Slow Climate Change
Rerouting the flight paths of commercial aircraft to minimize the condensation trails, or contrails, they leave behind would help slow global warming, even if
the new flight path is longer, according to research published today
. Contrails, thin clouds composed of ice crystals condensed from an aircraft's exhaust, can persist for 17 hours or more and are likely the single largest contributor to climate change
associated with aviation. They form when a plane passes through parts of the atmosphere that are very cold and moist, usually near high pressure systems. The new research shows that avoiding contrail formation has greater climate benefits than avoiding additional carbon dioxide emissions associated with slightly longer flight routes. For example, for a small aircraft that is predicted to form a contrail 20 miles long, an alternative path that adds less than 200 miles will have a smaller climate impact than the contrail. For a larger aircraft, which emits more CO2 per mile than a smaller plane, the alternative route is preferable if it adds less than 60 miles, according to researchers from the University of Reading.
18 Jun 2014:
Global Energy Systems Must
Prepare for Climate Change, Study Says
Power plants and energy systems around the world will experience potentially disastrous effects from climate change and should develop plans for dealing with those effects, according to a report released today
by the World Energy Council and European researchers. Long-term droughts, for example, could threaten water supplies needed to cool large power plants as they produce electricity, the report notes
. Many energy facilities are also lacking protection from floods, rising seas, and severe weather events — a problem highlighted by the 2011 Fukushima nuclear disaster. Strong global political action could have major impacts on the energy sector, the report says, especially if governments make a coordinated effort to invest in renewable and low-carbon energy and upgrades to power distribution grids.
17 Jun 2014:
Livestock Maps Highlight
Regions Prone to Disease and Pollution
A new mapping tool
shows the global distribution of cattle, pigs, and other livestock in high-resolution, 1-square-kilometer detail. Created by the International
Livestock Research Institute, the Food and Agriculture Organization of the United Nations, and British and Belgian researchers, the maps are the most detailed renditions ever produced of the planet's billions of cattle, pigs, sheep, goats, chickens, and ducks. They reveal several notable trends
, such as the high density of pigs in China, India's relatively high sheep and goat populations, and the affinity of the southeastern U.S. for chickens. Researchers say they will be powerful tools for tracking and predicting livestock-borne disease outbreaks, such as avian flu strains that have been linked to dense poultry markets. The maps could also predict where major livestock operations are most likely to harm the environment, researchers say. As livestock production increases, demands on land, water, and energy intensify, and so does pollution from livestock waste
13 Jun 2014:
U.S. Energy System Depicted
In New Mapping Tool from Federal Agency
The U.S. Energy Information Administration (EIA) recently launched a mapping system
that allows users to
explore the landscape of energy sources and power plant distribution across the nation. Among the most striking visualizations is this map showing the widespread distribution of natural gas power plants, marking the fossil fuel's growing use as an energy source. The U.S. had 1,714 natural gas power plants in 2012, accounting for 30 percent of the country's electricity generation, Vox reports
. Natural gas plants are easier to build and emit fewer pollutants and roughly half as much CO2 as coal-fired plants. The EIA maps depict numerous aspects of the U.S. power system, including the distribution of wind turbines, solar installations, nuclear power plants, coal-fired plants, hydropower stations, and pipelines. More energy maps
are available at Vox, or users can create their own with the EIA's tool.
09 Jun 2014:
Air Conditioning Can Raise
Urban Nighttime Temperature by 2 Degrees
Excess heat from air conditioners raises outdoor temperatures at night by nearly 2 degrees F (1 degree C), worsening the urban heat island effect and increasing cooling demands, according to research
from Arizona State University. Studying the Phoenix metropolitan area, researchers found that air conditioning systems pumped more waste heat into the air during the day, but the effect on near-surface temperatures was negligible. The same was not true for nighttime temperatures, however, when waste heat significantly boosts air temperatures because of nighttime atmospheric conditions. Air conditioning systems can consume more than 50 percent of total electricity
during extreme heat, the researchers note, and summertime extreme-heat days are projected to become more frequent and intense as a result of climate change. Redirecting waste heat from air conditioning systems to household appliances such as water heaters, for example, could help alleviate the problem, the scientists say. They project that such strategies would save at least 1,200 to 1,300 megawatt-hours of energy per day in the Phoenix metropolitan area alone.
Interview: The Small College That
Launched Fossil Fuel Divestment
When Stanford University announced in May that it would divest its endowment of coal mining companies, it was following the lead of a tiny college in rural Maine
that dubs itself “America’s environmental college.” A year and a half earlier, Stephen Mulkey, the president of Unity College
stood on a stage with Bill McKibben, the founder of 350.org and lead cheerleader for the divestment movement, to announce that his college would be the first institution of higher learning to rid its endowment of all fossil fuel holdings. In an interview with Yale Environment 360
, Mulkey, a climate scientist, talks about the ethical imperative behind the decision to divest, and his vision for, as he puts it, a re-engineering of the way the environmental sciences are taught.
03 Jun 2014:
Developing Countries Lead
Global Surge in Renewable Energy Capacity
The number of developing nations with policies supporting renewable energy has surged more than six-fold in just eight years, from 15 developing countries
in 2005 to 95 early this year, according to a report from REN21
, an international nonprofit renewable energy policy network. Those 95 developing nations today make up the vast majority of the 144 countries with renewable energy support policies and targets in place. The report credits
such policies with driving global renewable energy capacity to a new record level last year — 1,560 gigawatts, up 8.3 percent from 2012. More than one-fifth — 22 percent — of the world's power production now comes from renewable sources. Overall, renewables accounted for more than 56 percent of net additions to global power capacity in 2013, the report says. Although financial and policy support declined in the U.S. and some European countries, China, the U.S., Brazil, Canada, and Germany remained the top nations for total installed renewable power. China's new renewable power capacity surpassed new fossil fuel and nuclear capacity for the first time, the analysis found.
02 Jun 2014:
New U.S. Coal Plant Rules
Could Lead to a Steep Drop in Emissions
The Obama administration today unveiled a sweeping new plan
that aims to cut carbon dioxide emissions from the nation’s 600 coal-fired power plants by roughly a third. Gina McCarthy, administrator of the U.S. Environmental Protection Agency, said the new rules would give states maximum flexibility
to achieve the goal of reducing power plant emissions 30 percent below 2005 levels by 2030. Hundreds of coal-fired power plants are expected to close under the EPA plan. But rather than immediately shutting down plants, states would be allowed to reduce emissions by making changes across their electricity systems — by installing new wind and solar generation or energy-efficiency technology, continuing to expand the use of natural gas, and by starting or joining state and regional “cap and trade” programs. “There is no one-size-fits-all solution ... so each state’s path can be different,” said McCarthy. The proposed regulations could be held up by legal challenges. Obama administration officials said the rules would lift the U.S. into a clear global leadership position
on combating global warming.
30 May 2014:
New Battery Technology
Could Offer Cheap Renewable Energy Storage
New battery technology that uses cheaper and safer materials to store large amounts of energy may soon enable utility companies to use more renewable power,
Iron-chromium flow battery technology
MIT Technology Review. The new device is a type of flow battery, and it uses liquid materials that rely on iron-chromium chemical reactions to store energy. California-based startup Enervault, developer of the new battery, figured out how to use materials that had been tried in batteries decades ago; Enervault overcame a key technical challenge that had caused the earlier batteries to quickly degrade. The new battery is large — it can store one megawatt-hour of electricity, or enough to run 10,000 100-watt light bulbs for an hour — and the materials last more than 20 years, according to its developer. Although the battery is inefficient compared to conventional batteries — it loses 30 percent of the energy used to charge it — it is still economically viable, the company says. The iron-chromium flow battery costs 80 percent less than vanadium flow batteries, a competing technology. The batteries are currently in use at a small power plant near Modesto, California.
29 May 2014:
Electric Airplane Debut
Offers Hopes for Cutting Emissions
The aircraft manufacturing giant Airbus recently unveiled
a fully-electric aircraft which, if widely adopted, could reduce the aerospace industry's carbon
The recently debuted, fully-electric E-Fan
dioxide emissions by an order of magnitude. The E-Fan aircraft has two, 30-kilowatt electric motors powered by a series of lithium-ion batteries in the wings of the plane, as well as a 6-kilowatt electric motor in the wheel to provide extra power during takeoff and taxiing. Despite incorporating highly energy-efficient and aerodynamic design elements, however, the E-Fan has only a one-hour range and cannot leave the vicinity of the airport. Airbus says that future designs will rely on electric-hybrid engine technology and that by 2050 such airplanes should be able to accommodate 70 to 80 passengers on a three-hour flight. The plans were spurred, in part, by the European Union's Flight Path 2050
, which aims to reduce the aviation sector's nitrous oxide emissions by 90 percent, noise pollution by 65 percent, and carbon dioxide emissions by 75 percent by 2050. "It's a very different way of flying," said Jean Botti, a technology officer at Airbus Group, "absolutely no noise, no emissions."
23 May 2014:
Oil Drilling Permits
Issued for Key Area of Yasuni Park
The Ecuadorean government has issued permits to begin oil drilling
in a key area of the Yasuni Biosphere Reserve and National Park
, one of the most biodiverse places on the planet. Environment Minister Lorena
Tapia said the government had signed permits to begin preparations for drilling in the so-called ITT section of the park, which contains two uncontacted indigenous tribes; drilling itself could begin as early as 2016, the government said. Ecuador’s President, Rafeal Correa, had offered to ban drilling in large sections of the park if the international community raised $3.6 billion to compensate the country for leaving the oil in the ground. But after only $13 million was raised, Correa gave the green light to drilling, saying “the world has failed us.” Oil drilling has already taken place in some areas of the 6,500-square-mile park. As this Yale Environment 360 video
shows, Yasuni is home to a remarkable array of species, including roughly 400 species of fish, 600 species of birds, and thousands of species of vascular plants and trees.
19 May 2014:
India's New Prime Minister
Plans To Make A Major Push on Solar Energy
India's new government plans to bring electricity to the homes of its entire population of 1.2 billion within the next five years, largely through solar panel installations,
Bloomberg News reports
. Although nearly 400 million Indians do not have access to electricity, newly elected prime minister Narendra Modi, who won an overwhelming victory in last week's national vote, has pledged to enable every household to run at least one light bulb by 2019. If all goes well, household solar projects would allow every home to run two light bulbs, a solar cooker, and a television, one of Modi's energy advisers said. The plan follows an unfulfilled pledge from the previous administration to bring electricity to all homes by 2012. Modi, who pioneered India's first incentive program for large-scale solar projects when he was chief minister of Gujarat state, has made expanding solar a top priority because it has the potential to create jobs and supply power to millions of households, many of which are scattered throughout rural areas and not connected to the grid
. "We look upon solar as having the potential to completely transform the way we look at the energy space," said the energy adviser.
12 May 2014:
Global Renewable Energy Jobs
Grew by 14 Percent in 2013, Report Says
Renewable energy jobs grew by 14 percent to 6.5 million employees worldwide last year, led by the solar panel industry, according to a new report
from the International Renewable Energy Agency. Employing a
total of 2.6 million workers in renewable energy jobs, China led in hiring last year, followed by Brazil and the United States. The solar industry — spurred by increasing photovoltaic panel installations in Asia and falling prices — employed 2.27 million workers at the end of 2013, largely concentrated in China, the report said. The biofuel industry, with 1.45 million employees, and wind energy, with 830,000, were the second- and third-largest employers. Although policy changes in several countries have reduced wind energy installation jobs, operations and maintenance positions have experienced some growth, according to the report.
07 May 2014:
Stanford Drops Coal Stocks
From Its $18.7 Billion Endowment Portfolio
Stanford has become the first major U.S. university to divest its shares in coal-mining companies from its endowment funds, lending support to a growing nationwide movement calling for universities and
Fossil Free Stanford
pension funds to drop investments in fossil fuel companies. Citing guidelines that allow trustees to weigh whether “corporate policies or practices create substantial social injury” when choosing investments for the university's $18.7 billion endowment, the board decided, after five months of deliberation, to purge stakes in up to 100 companies worldwide that derive profits primarily from coal mining. A Stanford spokeswoman said that coal companies constitute a small fraction
of the university's total endowment investments, “but a small percentage is still a substantial amount of money." Board members said their decision was made partly because coal is the most carbon-intensive of any major fossil fuel and that less carbon-intensive energy sources are available.
02 May 2014:
Fracking May Induce Quakes at
Greater Distance than Previously Thought
Hydraulic fracturing and underground wastewater disposal may trigger earthquakes at tens of kilometers from the wells in which water is injected — a greater range than previously thought, according to new research from seismologists
. In one case, an earthquake
Fracking injection well in Louisiana
swarm in Oklahoma has been linked to a cluster of fracking injection wells up to 50 kilometers (31 miles) away, Cornell University researchers report. So-called "induced seismicity" — when human activity causes tremors in the earth's crust — is gaining attention as reports of earthquakes within the central and eastern U.S. have increased dramatically over the past few years. The rise coincides with increased hydraulic fracturing for oil and natural gas, and the injection of wastewater in deep disposal wells in many locations, including Colorado, Oklahoma, and Texas. According to the U.S. Geological Survey (USGS), an average rate of 100 earthquakes per year above a magnitude 3.0 occurred in the three years from 2010-2012, compared with an average rate of 21 events per year observed from 1967-2000.
29 Apr 2014:
Exxon Mobil Arctic Project
Possibly At Risk Over Russia Sanctions
Exxon Mobil's development of a Russian Arctic oil project valued at nearly $900 billion is at risk following recent U.S. sanctions on Russian officials as a result of Ukraine's ongoing political crisis, Bloomberg News reports
. Exxon Mobil has partnered with the Russian state-controlled oil company OAO Rosneft to drill an oil-rich geological structure known as Universitetskaya, which contains an estimated 9 billion barrels of oil, valued at $900 billion at current market prices. Rosneft's CEO, Igor Sechin, a longtime member of Russian President Vladimir Putin's inner circle, was sanctioned by the U.S. yesterday, exposing the partnership with Exxon Mobil to additional scrutiny. Exxon Mobil and Rosneft are set to invest an estimated $600 million in drilling at the site in the Kara Sea, which would make the project Exxon's most expensive to-date. A U.S. Treasury official said yesterday that U.S. companies can still do business with Rosneft, but some analysts say Russian companies could become wary of working with Western corporations in the future. Exxon said last week that the project is on schedule.
28 Apr 2014:
Economic Viability of Nuclear
Power Under Threat, Energy Group Says
Nuclear reactors in the U.S. need a boost — either through carbon taxes or regulations forcing coal-fired plants to slash emissions — or economic factors will force many to close, according to a report
from a non-profit group. Nuclear power — currently the only major zero-carbon, around-the-clock baseload power source — supplies 19 percent of U.S. electricity and is key to meeting President Obama's pledge to reduce emissions 17 percent below 2005 levels by 2020, according to the Center for Climate and Energy Solutions. The economic viability of the 100 nuclear reactors in the U.S. is worsening, the report says, because of the abundance of cheap natural gas and rising wind energy production. A carbon tax or some form of carbon trading — for instance, requiring coal-fired plants to purchase and blend their electricity output with nuclear power — will be essential to keeping nuclear plants from closing before the end of their lifespans, the report contends. Four power companies recently announced the early retirement of five nuclear reactors, which constitute more than 4 percent of U.S. nuclear capacity, the group says.
Five Questions for IPCC Chairman
On Future of Climate Change Action
The UN Intergovernmental Panel on Climate Change issued a report last month on steps the world can take
to avoid the worst impacts of future climate change. It was the final interim report before the IPCC’s major Fifth Assessment Report due to be released in October. Yale Environment 360
asked Rajendra K. Pachauri, who has served as IPCC chairman since 2002, five questions about the latest report and about the prospects that the international community will finally take decisive action to address climate change at talks scheduled in Paris in 2015.Read more.
18 Apr 2014:
Scale and Extent of Dam Boom
In China Is Detailed in Mapping Project
China is planning to build at least 84 major dams in its southwest region, as shown in a map from the Wilson Center, eventually boosting its hydropower capacity by more than 160 gigawatts. By next year China's capacity
will surpass Europe's, and by 2020 it's projected to be larger than that of the U.S. and Europe combined. An interactive map
shows the scale and number of major dams proposed, under construction, existing, and canceled. The dam rush is part of an ongoing effort by China to increase non-fossil energy sources to 11.4 percent of the country's total energy consumption — a goal that has gained urgency due to severe air pollution in many northern Chinese cities. However, the hydropower push is not without its own major environmental consequences
, the Wilson Center notes. The cascades of planned dams will submerge important corridors connecting tropical rainforests to the Tibetan Plateau that allow wildlife to migrate as temperatures rise.
15 Apr 2014:
Possible with Advances in Semiconductors
New semiconductor technology is advancing the development of house windows that could double as solar panels, according to scientists
from Los Alamos National Lab and Italy. Their research into so-called
Solar-harvesting materials under UV light
"quantum dots" — ultra-small bits of semiconductors that transmit energy extremely efficiently and can be tuned toward specific colors — shows that quantum dots can be used in transparent materials to harvest sunlight with efficiencies comparable to standard solar panels. When highly transparent materials are embedded with quantum dots, they are known as luminescent solar concentrators (LSCs); the structures can absorb sunlight and re-radiate it at longer wavelengths directed toward the edge of the slab, where the energy is collected by a solar cell. In tests using large LSC slabs (sized in tens of centimeters), researchers reported harvesting photons at roughly 10 percent efficiency. Typical photovoltaic solar panels have an average efficiency of about 15 percent.