Imagine a time when every rooftop in the world has a solar panel on it. Looking at Google Earth, all the rooftops you see would have panels. Panels would provide power or heat, or both. I am calling this vision "no rooftop left behind." That phrase may ring familiar to Americans who recall the "no child left behind" legislation in 2001, which mandated educational standards for children—everyone is included, no one left out. Let's think the same way about rooftops. Let’s adopt this phrase into our common lexicon. Let’s make this a rallying call for the future of our planet.Many are coming to the conclusion that putting solar panels on virtually all rooftops worldwide is entirely possible. As just one example, UBS Bank, in a 2013 research note, wrote: "Purely based on economics, we believe almost every family home and every commercial rooftop in Germany, Italy, and Spain should be equipped with a solar system by the end of this decade."
Recently, NRG Chief Executive David Crane was quoted by Reuters as saying, “What really gets me excited in the morning is that there are 50 million American buildings that should have solar PV on them.”
By one estimate, there are already at least 15-20 million rooftops worldwide bearing solar power (PV) panels, primarily residential and commercial buildings. There are also more than 90 million rooftops worldwide with solar hot water/heating systems installed, the vast majority in China.
There must be well over 1 billion rooftops worldwide, so there is still a long way to go.
Growing Markets
Global solar power (PV) capacity stood at about 140 gigawatts (GW) in 2013. (For comparison, all the nuclear power plants in the world total about 370 GW.) Scenario projections for solar PV capacity by 2030 by a variety of organizations range up to 1800 GW or more. (See GFR Chapter 6.) Even if the annual market were to stay constant at 2013 levels, the world would reach about 800 GW by 2030. These numbers would represent a 6-fold to 12-fold increase from 2013, with likely a similar scale increase in the number of solar PV rooftops. (Also depends on the future balance of rooftop vs. “utility-scale” ground-mounted systems, and the balance between commercial and residential rooftops. Some believe that utility-scale systems will take a growing market share.)
And a doubling or tripling of rooftops with solar hot water/heating by 2030 would not be inconceivable given China’s ambitious future targets for solar hot water and other global market trends.
Roughly 40 GW of solar PV was added in 2013, a new market record. That translates into perhaps 3-4 million new rooftops, something like 400 rooftops per hour. In 2012, about half of all global investment in renewable energy went to solar PV.
Most don’t realize how quickly the market for solar PV has grown in recent years. Ten years ago, when the annual market for grid-tied solar PV was only 1 GW, no one imagined a market 40 times larger so quickly. China, Japan, and the U.S. are now the leaders, usurping the positions of former leaders Germany and Italy. The Chinese market was 7-9 GW in 2013, with 12 GW planned for 2014. Japan was close behind at 6 GW. The U.S. was above 4 GW.
Policy-Led Growth
The growth of solar PV has been driven by a variety of policies around the world over the past twenty years. Historically, most policy support has been in the form of capital subsidies, guaranteed prices from feed-in tariffs, or net metering to allow consumers to offset consumption. One of the first subsidy policies was Japan's "Sunshine" program of 1993. Later, Germany’s feed-in tariff applied to solar PV. (Feed-in tariffs now exist in over 80 countries around the world.) The U.S. has had a variety of state-level subsidies, and since 2005, a national investment tax credit. China came late to the game, but policies of recent years, including a new 2013 feed-in tariff, have brought its domestic market from practically zero to being the global leader.
These policies of the past twenty years have been instrumental in bringing about major economies of scale in manufacturing, along with technology improvements, that together have lowered solar power costs enormously. In recent years, the cost of a solar PV module has fallen below one dollar (US) per watt, which back in the 1980s was the considered the "holy grail." Experts used to say that once solar PV falls below one dollar, “the game is over, solar will be everywhere." Well, that time has arrived. (And accounting for inflation, today’s cost is actually well below one 1990 dollar.)
City governments around the world have also been supporting rooftop solar for more than a decade with a variety of policies, subsidies, and targets. There are now hundreds of such cities and local communities. As one example, the city of Iida in Japan targets 40% of all rooftops with solar PV by 2030, up from the current 7%, which itself is a doubling from 3.5% in 2011. Several cities in China target high shares of buildings (50% or more) with rooftop solar hot water. Some cities in Europe have policies or mandates for solar PV on new construction.
The Competitiveness of Solar PV
Many talk of "grid parity" for solar PV. Generally, “grid parity” is accepted by most to mean equivalence of solar PV generation costs with retail electricity prices. However, this concept can be misleading or distorted due to subsidies and a variety of electricity-market practices and rules (i.e., differential prices across customer classes, seasonal pricing, and net metering rules). If customers face time-of-use pricing, or prices based dynamically on grid conditions, then grid parity may well exist at some times but not others. Furthermore, financial experts point out that “cost of electricity” metrics, including grid parity, are not as important to investors as rate-of-return financial metrics. (For more on grid parity, see "Solar PV" in Chapter 6 of the GFR and associated endnotes.)
The International Energy Agency, in its 2013 World Energy Outlook, points out that grid parity may not mean economic competitiveness because solar generators must still pay their share of fixed grid costs, even if most of their power is self-generated. However, that is only true under a policy model in which "stranded assets"—the generation, transmission and/or distribution infrastructure rendered unneeded by the growth of distributed solar power—must still be paid for by all consumers equally. Other policy models may allocate costs differently, resulting in different winners and losers. In countries with fast growing power demand, the issue may be mute, as solar PV slows the need for more grid investment—part of the “value” of distributed generation.
So questions arise like: is it socially equitable for a declining base of customers to pay increasing rates to cover fixed infrastructure costs? Should self-generators bear the full grid costs? And how should electricity markets price and value the attributes of "flexibility," "reliability," "back-up," and "capacity,” rather than just “kilowatt hours produced”?
Still, there is no dispute that in many jurisdictions today, in places with high electricity prices and good solar resources, grid parity is already being reached, even without subsidies. Spain and Hawaii are good examples. Many experts envision expanding "waves" of grid parity reaching a growing number of jurisdictions around the world in the coming 5-10 years. Rooftop solar will become competitive without subsidies in a growing number of locations around the world, provided that the policy challenges ahead are addressed.
Innovations and Challenges Ahead
The main innovations and challenges ahead for rooftop solar relate not to the technologies themselves, but to innovations in business and finance, policy, and integration.
1. New business and finance models. Historically, rooftop solar meant outright purchases by building owners, requiring large capital outlays up-front. However, over the past several years, a variety of other models have emerged, such as leasing, fee-for-service, and pre-paid, which allow customers to pay nominal monthly amounts. Some models even allow renters rather than owners to choose solar, and many of these new innovations have been gaining mainstream acceptance by commercial financiers.
A growing variety of new "energy service company" models promise new forms of financial viability, from third-party building-level energy managers, to micro-utilities, district-energy providers, community-owned systems, cooperatives, and special-purpose entities. (For more on business and finance models, see Chapter 3 of the GFR.)
2. A new generation of power-sector policies and market rules. Policies must now shift from purely cost- or price-based support for renewables to defining new rules and market structures for electricity. In some jurisdictions, there is already some urgency, as established power companies suffer large financial loses on generation and grid assets due to the influx of renewable power. Those loses are leading to growing resistance to renewables by some of these companies, not primarily for technical reasons, but for financial ones. The required transitions implied by such financial loses must be addressed, either by the companies themselves, or through policy. (The GFR discusses the question in Chapter 3: “Will utilities lead, follow, push-back, or perish?”)
Policies must maintain grid access and pricing rules for solar, but there are many variations possible. “Net metering” policies are spreading, but differ widely. Net metering policies essentially allow for outflows of power to the grid—“grid access”—and set one buying price for incoming power and one selling price for outgoing power. Depending on jurisdiction, the selling price may be the same or lower than the buying price. Solar power production can offset or exceed local consumption, but only within established market rules.
Some net metering regimes give full retail price for outflows of solar power, others only wholesale prices. Some cap generation at the level of consumption. Other regimes allow surplus generation beyond the level of consumption, and thus the possibility of earning a monthly profit from the utility.
Feed-in tariffs have played the same function as net metering, providing guaranteed prices and access. Some see a transition underway from feed-in tariffs to net metering, although some solar advocates are loath to suggest the end of feed-in tariffs. In any event, policy evolution, not termination, will be necessary. In jurisdictions without either a feed-in tariff or net metering, access to the grid is prevented, hindering solar growth even in the face of clear grid parity. Spain in recent years has been a classic case, after its feed-in tariffs expired.
Policy must also support new business models, for example "peer-to-peer" energy sales among neighbors, based on nominal “wheeling” charges for use of local wires. London and Sydney, for example, are considering streamlined rules and procedures, and modest wire fees, to support peer-to-peer sales.
3. Materials and systems integration. In 2012, only about 1% of the global solar PV market was for so-called “building-integrated” PV. However, the integration of solar PV into building materials, such as roofing materials and glass facades, represents an important innovation trend. Another trend is for energy-service companies or solar installers to offer systems with batteries packaged together with solar. Such systems are becoming common in China and India, with growing demand for them elsewhere. [14] (For more on integration with buildings, see “Buildings” and “Building Integrated PV” sections of Chapter 2 of the GFR.)
Objections and Myths
Finally, there are a few common misconceptions about solar that should be addressed, for the naysayers who argue we'll never get close to "no rooftop left behind."
1. “The lights will go out.” Contrary to the conventional thinking of past decades, there are several viable options for balancing high shares of renewable energy on power grids. And allof these options are already being used somewhere around the world today to manage the variability of wind and solar power.
Demand flexibility, or “demand response,” is one such important option. Large numbers of customers, especially industrial and commercial, agree to allow some of their load to be modulated using smart controls, over periods ranging from minutes to hours, in return for lucrative monthly payments. Such "aggregation" of many such customers allows for a large capacity of demand to be varied in real time in response to variations in renewable power supply. Already, almost 10% of the entire capacity of the U.S. electric grid is harnessed as demand response, creating the potential for enormous flexibility.
Another option is natural gas peaking turbines. These are very cheap in terms of their capital costs, several times cheaper than a coal power plant or a wind farm, so the costs of having them sit unused are modest. Their primary cost comes from the cost of the natural gas fuel itself, which is only incurred when the turbines are needed.
Conventional power plants like coal and nuclear can also be adjusted to enable them to “ramp up/down" and "cycle” their power output to follow variable renewable generation, although not without additional costs and accelerated maintenance. Ramping can be scheduled based on sophisticated day-ahead weather forecasting models that can predict future renewable power output, which is already being done in several jurisdictions. Renewables are variable but they are not unpredictable.
Several countries or states are already facing the “grid-integration and balancing” challenge as shares of variable renewables rise to levels above 30%, and even higher at peak moments. Germany is the best example. In Germany in 2013, solar and wind power provided up to 60% of the country’s (instantaneous) peak power demand on some days, a new record. (And 36% averaged over a whole day.) A 2013 Greentech Media article covering this fact was titled: “Germany Hits 59% Renewable Peak, Grid Does Not Explode.” (For more on options for managing variability, see "Electric Power Grids" in Chapter 2 of the GFR. And for links to dozens of articles and references on this subject, see my new Power-Grid Integration page on the information part of this web site.)
2. “Not enough raw materials.” This is an important issue but I won’t dispel it here, because it could ultimately prove true. But that would also spell the end of our industrial civilization as we know it. Given that there are more than a billion cars in the world today, a billion rooftop solar panels pales in comparison with the materials needed for the transport sector alone. The hope may come in advanced recycling techniques, but this is a problem not unique to renewables or solar.
3. “No power at night.” The fact that solar only produces power during daylight is mitigated by several factors. First, power grids experience high "peak" loads during the daytime, which can be met by solar, while nighttime loads are much lower and can be met by other generation. For example, air conditioning loads are a significant source of peak load in summer in warm climates, and correspond exactly with the time of day that solar power is producing the most power. Second, solar power can be stored during the daytime and used in the evening, on a daily cycle using pumped hydro storage or other forms of energy storage. (Pumped hydro capacity already exists in many countries. And concentrating solar thermal power (CSP) plants can store their daytime power in the form of heat, and use the heat in the evenings to continue to generate power for several hours.)
4. "Cloudy days.” First, solar does produce electricity even on cloudy days, just not as much. So it’s really a question of economics and nothing else. Either less energy is produced or more panels are needed to make up for the losses due to clouds. With the costs of solar panels falling precipitously these past years, solar profitability even in cloudy climates will improve. Rooftop area may become a limiting factor, but eventually it should become profitable to install panels on the sides of buildings, given cheap panels and high-volume adoption of building-integrated construction materials and practices (like solar glass for building sides). Second, cloud-induced variation can be predicted in advance and managed (see #1 above on keeping the lights on). Third, over a wider geographical region interconnected through transmission grids, solar balances out as passing clouds or storms reduce power only in selected spots, while the average output across the whole grid remains constant.
The Vision
Let me clear that I'm not saying that solar power is a panacea. There are many other renewable energy technologies that are equally important to the future. Biomass power and heat, especially in northern latitudes during wintertime, is an essential part of the future. Northern European countries already make strong use of biomass for energy. Wind, geothermal, hydro, and concentrating solar thermal power (CSP) all have an important role in power grids of the future, including in balancing and complementing solar. Many believe that we can approach 100% of our electricity from renewable energy in the future using these sources, coupled with appropriate power-grid balancing and management options. (See Chapters 1, 2 and 6 of the GFR.)
Still, solar presents a singular opportunity for distributed energy, for autonomy, for communities, and for entire cities. Most would allow that future energy systems will be a combination of centralized and decentralized, along with intermediate levels like district energy systems. More scenarios and visions are painting the decentralized part of this picture. For example, in the book by Amory Lovins/Rocky Mountain Institute “Reinventing Fire” (2011), one scenario shows a U.S. electricity system in 2050 that gets 80% of its power from renewables, and half of those renewables are integrated into an interlinked network of micro-grids with distributed energy resources.
And solar presents the opportunity for visions like "no rooftop left behind." Many people actually envision solar on all available surfaces, like parking areas, highways, and other public structures. Looking from Google Earth, all the rooftops we see would have panels for electricity and/or heating. Indeed, 20 years from now, a rooftop without solar on it would seem strange, almost "naked," to the sensibilities of the day. Let’s make "no rooftop left behind" a rallying call for the future of our planet.
Click here to read the full article on Eric Martinot's website
Recently, NRG Chief Executive David Crane was quoted by Reuters as saying, “What really gets me excited in the morning is that there are 50 million American buildings that should have solar PV on them.”
By one estimate, there are already at least 15-20 million rooftops worldwide bearing solar power (PV) panels, primarily residential and commercial buildings. There are also more than 90 million rooftops worldwide with solar hot water/heating systems installed, the vast majority in China.
There must be well over 1 billion rooftops worldwide, so there is still a long way to go.
Growing Markets
Global solar power (PV) capacity stood at about 140 gigawatts (GW) in 2013. (For comparison, all the nuclear power plants in the world total about 370 GW.) Scenario projections for solar PV capacity by 2030 by a variety of organizations range up to 1800 GW or more. (See GFR Chapter 6.) Even if the annual market were to stay constant at 2013 levels, the world would reach about 800 GW by 2030. These numbers would represent a 6-fold to 12-fold increase from 2013, with likely a similar scale increase in the number of solar PV rooftops. (Also depends on the future balance of rooftop vs. “utility-scale” ground-mounted systems, and the balance between commercial and residential rooftops. Some believe that utility-scale systems will take a growing market share.)
And a doubling or tripling of rooftops with solar hot water/heating by 2030 would not be inconceivable given China’s ambitious future targets for solar hot water and other global market trends.
Roughly 40 GW of solar PV was added in 2013, a new market record. That translates into perhaps 3-4 million new rooftops, something like 400 rooftops per hour. In 2012, about half of all global investment in renewable energy went to solar PV.
Most don’t realize how quickly the market for solar PV has grown in recent years. Ten years ago, when the annual market for grid-tied solar PV was only 1 GW, no one imagined a market 40 times larger so quickly. China, Japan, and the U.S. are now the leaders, usurping the positions of former leaders Germany and Italy. The Chinese market was 7-9 GW in 2013, with 12 GW planned for 2014. Japan was close behind at 6 GW. The U.S. was above 4 GW.
Policy-Led Growth
The growth of solar PV has been driven by a variety of policies around the world over the past twenty years. Historically, most policy support has been in the form of capital subsidies, guaranteed prices from feed-in tariffs, or net metering to allow consumers to offset consumption. One of the first subsidy policies was Japan's "Sunshine" program of 1993. Later, Germany’s feed-in tariff applied to solar PV. (Feed-in tariffs now exist in over 80 countries around the world.) The U.S. has had a variety of state-level subsidies, and since 2005, a national investment tax credit. China came late to the game, but policies of recent years, including a new 2013 feed-in tariff, have brought its domestic market from practically zero to being the global leader.
These policies of the past twenty years have been instrumental in bringing about major economies of scale in manufacturing, along with technology improvements, that together have lowered solar power costs enormously. In recent years, the cost of a solar PV module has fallen below one dollar (US) per watt, which back in the 1980s was the considered the "holy grail." Experts used to say that once solar PV falls below one dollar, “the game is over, solar will be everywhere." Well, that time has arrived. (And accounting for inflation, today’s cost is actually well below one 1990 dollar.)
City governments around the world have also been supporting rooftop solar for more than a decade with a variety of policies, subsidies, and targets. There are now hundreds of such cities and local communities. As one example, the city of Iida in Japan targets 40% of all rooftops with solar PV by 2030, up from the current 7%, which itself is a doubling from 3.5% in 2011. Several cities in China target high shares of buildings (50% or more) with rooftop solar hot water. Some cities in Europe have policies or mandates for solar PV on new construction.
The Competitiveness of Solar PV
Many talk of "grid parity" for solar PV. Generally, “grid parity” is accepted by most to mean equivalence of solar PV generation costs with retail electricity prices. However, this concept can be misleading or distorted due to subsidies and a variety of electricity-market practices and rules (i.e., differential prices across customer classes, seasonal pricing, and net metering rules). If customers face time-of-use pricing, or prices based dynamically on grid conditions, then grid parity may well exist at some times but not others. Furthermore, financial experts point out that “cost of electricity” metrics, including grid parity, are not as important to investors as rate-of-return financial metrics. (For more on grid parity, see "Solar PV" in Chapter 6 of the GFR and associated endnotes.)
The International Energy Agency, in its 2013 World Energy Outlook, points out that grid parity may not mean economic competitiveness because solar generators must still pay their share of fixed grid costs, even if most of their power is self-generated. However, that is only true under a policy model in which "stranded assets"—the generation, transmission and/or distribution infrastructure rendered unneeded by the growth of distributed solar power—must still be paid for by all consumers equally. Other policy models may allocate costs differently, resulting in different winners and losers. In countries with fast growing power demand, the issue may be mute, as solar PV slows the need for more grid investment—part of the “value” of distributed generation.
So questions arise like: is it socially equitable for a declining base of customers to pay increasing rates to cover fixed infrastructure costs? Should self-generators bear the full grid costs? And how should electricity markets price and value the attributes of "flexibility," "reliability," "back-up," and "capacity,” rather than just “kilowatt hours produced”?
Still, there is no dispute that in many jurisdictions today, in places with high electricity prices and good solar resources, grid parity is already being reached, even without subsidies. Spain and Hawaii are good examples. Many experts envision expanding "waves" of grid parity reaching a growing number of jurisdictions around the world in the coming 5-10 years. Rooftop solar will become competitive without subsidies in a growing number of locations around the world, provided that the policy challenges ahead are addressed.
Innovations and Challenges Ahead
The main innovations and challenges ahead for rooftop solar relate not to the technologies themselves, but to innovations in business and finance, policy, and integration.
1. New business and finance models. Historically, rooftop solar meant outright purchases by building owners, requiring large capital outlays up-front. However, over the past several years, a variety of other models have emerged, such as leasing, fee-for-service, and pre-paid, which allow customers to pay nominal monthly amounts. Some models even allow renters rather than owners to choose solar, and many of these new innovations have been gaining mainstream acceptance by commercial financiers.
A growing variety of new "energy service company" models promise new forms of financial viability, from third-party building-level energy managers, to micro-utilities, district-energy providers, community-owned systems, cooperatives, and special-purpose entities. (For more on business and finance models, see Chapter 3 of the GFR.)
2. A new generation of power-sector policies and market rules. Policies must now shift from purely cost- or price-based support for renewables to defining new rules and market structures for electricity. In some jurisdictions, there is already some urgency, as established power companies suffer large financial loses on generation and grid assets due to the influx of renewable power. Those loses are leading to growing resistance to renewables by some of these companies, not primarily for technical reasons, but for financial ones. The required transitions implied by such financial loses must be addressed, either by the companies themselves, or through policy. (The GFR discusses the question in Chapter 3: “Will utilities lead, follow, push-back, or perish?”)
Policies must maintain grid access and pricing rules for solar, but there are many variations possible. “Net metering” policies are spreading, but differ widely. Net metering policies essentially allow for outflows of power to the grid—“grid access”—and set one buying price for incoming power and one selling price for outgoing power. Depending on jurisdiction, the selling price may be the same or lower than the buying price. Solar power production can offset or exceed local consumption, but only within established market rules.
Some net metering regimes give full retail price for outflows of solar power, others only wholesale prices. Some cap generation at the level of consumption. Other regimes allow surplus generation beyond the level of consumption, and thus the possibility of earning a monthly profit from the utility.
Feed-in tariffs have played the same function as net metering, providing guaranteed prices and access. Some see a transition underway from feed-in tariffs to net metering, although some solar advocates are loath to suggest the end of feed-in tariffs. In any event, policy evolution, not termination, will be necessary. In jurisdictions without either a feed-in tariff or net metering, access to the grid is prevented, hindering solar growth even in the face of clear grid parity. Spain in recent years has been a classic case, after its feed-in tariffs expired.
Policy must also support new business models, for example "peer-to-peer" energy sales among neighbors, based on nominal “wheeling” charges for use of local wires. London and Sydney, for example, are considering streamlined rules and procedures, and modest wire fees, to support peer-to-peer sales.
3. Materials and systems integration. In 2012, only about 1% of the global solar PV market was for so-called “building-integrated” PV. However, the integration of solar PV into building materials, such as roofing materials and glass facades, represents an important innovation trend. Another trend is for energy-service companies or solar installers to offer systems with batteries packaged together with solar. Such systems are becoming common in China and India, with growing demand for them elsewhere. [14] (For more on integration with buildings, see “Buildings” and “Building Integrated PV” sections of Chapter 2 of the GFR.)
Objections and Myths
Finally, there are a few common misconceptions about solar that should be addressed, for the naysayers who argue we'll never get close to "no rooftop left behind."
1. “The lights will go out.” Contrary to the conventional thinking of past decades, there are several viable options for balancing high shares of renewable energy on power grids. And allof these options are already being used somewhere around the world today to manage the variability of wind and solar power.
Demand flexibility, or “demand response,” is one such important option. Large numbers of customers, especially industrial and commercial, agree to allow some of their load to be modulated using smart controls, over periods ranging from minutes to hours, in return for lucrative monthly payments. Such "aggregation" of many such customers allows for a large capacity of demand to be varied in real time in response to variations in renewable power supply. Already, almost 10% of the entire capacity of the U.S. electric grid is harnessed as demand response, creating the potential for enormous flexibility.
Another option is natural gas peaking turbines. These are very cheap in terms of their capital costs, several times cheaper than a coal power plant or a wind farm, so the costs of having them sit unused are modest. Their primary cost comes from the cost of the natural gas fuel itself, which is only incurred when the turbines are needed.
Conventional power plants like coal and nuclear can also be adjusted to enable them to “ramp up/down" and "cycle” their power output to follow variable renewable generation, although not without additional costs and accelerated maintenance. Ramping can be scheduled based on sophisticated day-ahead weather forecasting models that can predict future renewable power output, which is already being done in several jurisdictions. Renewables are variable but they are not unpredictable.
Several countries or states are already facing the “grid-integration and balancing” challenge as shares of variable renewables rise to levels above 30%, and even higher at peak moments. Germany is the best example. In Germany in 2013, solar and wind power provided up to 60% of the country’s (instantaneous) peak power demand on some days, a new record. (And 36% averaged over a whole day.) A 2013 Greentech Media article covering this fact was titled: “Germany Hits 59% Renewable Peak, Grid Does Not Explode.” (For more on options for managing variability, see "Electric Power Grids" in Chapter 2 of the GFR. And for links to dozens of articles and references on this subject, see my new Power-Grid Integration page on the information part of this web site.)
2. “Not enough raw materials.” This is an important issue but I won’t dispel it here, because it could ultimately prove true. But that would also spell the end of our industrial civilization as we know it. Given that there are more than a billion cars in the world today, a billion rooftop solar panels pales in comparison with the materials needed for the transport sector alone. The hope may come in advanced recycling techniques, but this is a problem not unique to renewables or solar.
3. “No power at night.” The fact that solar only produces power during daylight is mitigated by several factors. First, power grids experience high "peak" loads during the daytime, which can be met by solar, while nighttime loads are much lower and can be met by other generation. For example, air conditioning loads are a significant source of peak load in summer in warm climates, and correspond exactly with the time of day that solar power is producing the most power. Second, solar power can be stored during the daytime and used in the evening, on a daily cycle using pumped hydro storage or other forms of energy storage. (Pumped hydro capacity already exists in many countries. And concentrating solar thermal power (CSP) plants can store their daytime power in the form of heat, and use the heat in the evenings to continue to generate power for several hours.)
4. "Cloudy days.” First, solar does produce electricity even on cloudy days, just not as much. So it’s really a question of economics and nothing else. Either less energy is produced or more panels are needed to make up for the losses due to clouds. With the costs of solar panels falling precipitously these past years, solar profitability even in cloudy climates will improve. Rooftop area may become a limiting factor, but eventually it should become profitable to install panels on the sides of buildings, given cheap panels and high-volume adoption of building-integrated construction materials and practices (like solar glass for building sides). Second, cloud-induced variation can be predicted in advance and managed (see #1 above on keeping the lights on). Third, over a wider geographical region interconnected through transmission grids, solar balances out as passing clouds or storms reduce power only in selected spots, while the average output across the whole grid remains constant.
The Vision
Let me clear that I'm not saying that solar power is a panacea. There are many other renewable energy technologies that are equally important to the future. Biomass power and heat, especially in northern latitudes during wintertime, is an essential part of the future. Northern European countries already make strong use of biomass for energy. Wind, geothermal, hydro, and concentrating solar thermal power (CSP) all have an important role in power grids of the future, including in balancing and complementing solar. Many believe that we can approach 100% of our electricity from renewable energy in the future using these sources, coupled with appropriate power-grid balancing and management options. (See Chapters 1, 2 and 6 of the GFR.)
Still, solar presents a singular opportunity for distributed energy, for autonomy, for communities, and for entire cities. Most would allow that future energy systems will be a combination of centralized and decentralized, along with intermediate levels like district energy systems. More scenarios and visions are painting the decentralized part of this picture. For example, in the book by Amory Lovins/Rocky Mountain Institute “Reinventing Fire” (2011), one scenario shows a U.S. electricity system in 2050 that gets 80% of its power from renewables, and half of those renewables are integrated into an interlinked network of micro-grids with distributed energy resources.
And solar presents the opportunity for visions like "no rooftop left behind." Many people actually envision solar on all available surfaces, like parking areas, highways, and other public structures. Looking from Google Earth, all the rooftops we see would have panels for electricity and/or heating. Indeed, 20 years from now, a rooftop without solar on it would seem strange, almost "naked," to the sensibilities of the day. Let’s make "no rooftop left behind" a rallying call for the future of our planet.
Click here to read the full article on Eric Martinot's website
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