Medium: Living in a carbon bubble

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Most of us live with high carbon dioxide (CO2) levels in our offices, bedrooms, classrooms, and cars without ever thinking about it. But recent double-blind studies suggest that CO2 exposure can reduce cognitive and decision-making performance dramatically — by 50% or more at common indoor levels. This is one of the most under-discussed impacts of climate change: It makes us all stupid. I monitored CO2 levels in my bedroom for the last 2 years, and the results are stunning.

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Stuff I didn’t know about Saudi Arabia (KAUST Solar Future Symposium)

Zooming into the King Abdullah University of Science and Technology (KAUST)

Zooming into the King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia

I’ve spent the past week in Saudi Arabia at the Solar Future 2015 Symposium, an annual conference on the basic science, devices, and systems of solar energy, hosted by the King Abdullah University of Science and Technology (KAUST). KAUST is one of the world’s newest universities. It was established in 2009, but its massive endowment—$20B today, a small step from Harvard/Yale/Stanford-land—has accelerated its progress toward its stated goal of becoming a global “destination for scientific and technological education and research” and its unstated goal of becoming a gushing geyser of Nature and Science papers.

Nestled in the welcoming desert of western Saudi Arabia, KAUST is modeled after Caltech, which is a small technical school in the welcoming desert of the western U.S. It even borrowed Caltech’s president in 2013.

KAUST only offers graduate education (900 master’s and PhD students today). Notably, 70% of its students are international (4% from the U.S.). Even more notably, 37% are women. All this in a country where alcohol is forbidden (for some reason, grape juice without preservatives is in high demand…); books and movies are censored (KAUST has the only movie theater in the entire country); and women can’t travel without a male chaperone, drive a car, try on clothes while shopping, or even interact with single men. That said, KAUST is far more liberal than the rest of the country, so much so that entry and exit must be controlled by 2 stages of security checkpoints—lest some un-abaya-ed female driver wander off campus and confuse the hell out of the Mutaween (religious police). KAUST is the very definition of a campus community: Everyone in the community lives and eats and works and plays on campus—there’s simply nothing else around except desert.

Would you care for some non-alcoholic grape drink?

Care for some non-alcoholic grape drink?

Of all the things I observed in Saudi Arabia—admittedly not much besides KAUST—I was most impressed and dismayed by the campus itself, which seems to be a collection of ultra-modern facilities with no one around to use them. Everything is new and shiny, from the rec center to the research labs to the central walkway (“the Spine”) to the campus library. Yet the whole place feels deserted; even at noon on a Tuesday, our tiny conference delegation far outnumbered the few KAUST community members in sight.

The research facilities are incredibly lavish; it’s almost as if the funding flows straight out of the Arabian desert. I can’t think of a single piece of equipment for nanoscale fabrication and characterization that they DIDN’T have. Here’s a quick snapshot of the shared labs at KAUST and the Solar & Photovoltaics Engineering Research Center (SPERC):

  • Visualization facility with 3-D interactive and immersive displays
  • Nanoscale imaging facility with 7 TEMs (including 5 Titans!) and 7 SEMs (including 4 FIBs)
  • Nanofab facility with class 100 cleanrooms
    • 6 Dimatix inkjet printers
    • 4 ellipsometers
    • 30 gloveboxes in one room, with integrated spin-coaters, e-beam evaporators, thermal evaporators, sputtering systems, and myriad characterization tools
    • Many, many laser labs

Take my word for it: These capabilities are enough to make materials and device researchers drool all over their bunny suits (and this is only what I saw in person!).

Hella gloveboxes.

Hella gloveboxes.

The symposium itself was fascinating, with talks by academics and industryfolk on the full spectrum of topics relevant to solar energy, from the photophysics of bulk heterojunctions to the newest developments in crystalline silicon, from power electronics to energy storage. It was a small single-track conference (similar to a Gordon conference)—meaning there was only one talk, poster session, or event happening at once, and everyone stayed together the whole time, including at every meal. I really like this type of conference: It leads to more continuity, in the form of deeper conversations that bleed over from hour to hour and from day to day.

Among the many topics of the week, I had interesting chats about silicon feedstock and material scaling limits with Bruno Ceccaroli, whether quantum dot solar cells have a future with Victor Klimov (Los Alamos), solar’s role in combatting climate change with Greg Wilson (NREL), why fracture energy matters for solar cells with Reinhold Dauskardt (Stanford), and the future potential of perovskites with pretty much everyone, including my esteemed friends and colleagues at MIT (Sam Stranks) and at Stanford (Tomas Leijtens and Nick Rolston).

All in all, I’d call it a successful trip—especially since Saudi Arabia doesn’t issue tourist visas, which means I probably won’t get another opportunity to visit unless I take a job delivering new TEMs to KAUST.

I’ll leave you with a few selfies.

KAUST's Hoover Tower

KAUST’s Hoover Tower

The Grand Mosque

The Grand Mosque

Caught in C60

Caught in C60

With Sam, Nick, and Tomas

With Sam, Nick, and Tomas

Selfie with Sam.

Selfie with Sam.

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Milano e Torino

I just got back from a 10-day trip to northern Italy, a combination of work and play. The work part was for a research workshop with Eni, a gigantic company you’ve probably never heard of unless you have a thing for 6-legged dogs.

Eni and its six-legged dog.

The 6 legs represent the 4 wheels of a car and the 2 legs of its driver.

As a token of goodwill and everlasting funding (haha NOT), Eni shipped us a human-sized stuffed dog-dragon-thing.

As a token of goodwill and everlasting research funding (haha NOT), Eni shipped us a human-sized stuffed dog-dragon-thing. It has many uses, including taking up office space and scaring undergrads.

Eni is the Italian national oil and gas company, known by some as “the state within the state” for its outsized influence in Italian politics. The company is trying to reinvent itself as an “energy company,” drilling for crude oil in North Africa with one hand while funding solar research internally and at MIT with the other.

It’s a tough balancing act. Oil supermajors (Exhibit 1: Exxon) aren’t particularly well known for believing in human-caused climate change, much less supporting a wholesale shift away from their hydrocarbon lifeblood***. It’s not clear to me whether their support of renewable energy research is merely a good PR move or reflects a genuine desire to save us all (OR perhaps just a hedge in case the world actually decides to do something about climate change). The real answer is probably (d) all of the above.

***Although it’s noteworthy (but not altogether surprising) that a few small oil companies—BP, Shell, Eni, Total, and Statoil—recently urged the U.N. to place a price on carbon.

In any case, Eni seems to be taking one small step in the right direction. During our visit to Eni’s headquarters in San Donato (just south of Milan), CTO Roberto Casula at least used all the right words in talking about climate change: that we need to start the low-carbon transition today, that an investment in renewables is an investment in the future, that Eni needs to become not just an oil company but an energy company, etc. And Eni has its own team of 20 researchers working on solar cells. They do great work on polymer PV, but I can’t help but laugh/cry when an “energy” company with 85,000 employees dedicates just 0.02% of its workforce to developing a key part of the future energy system. Maybe I’m too sentimental.

After the workshop in San Donato, I did some solo traveling, visiting the World Expo in Milan and posing as an Italian in Turin (~1.5 hours west of Milan by train) with Francesca, an MIT friend and colleague who grew up there and was kind enough to show me around her hometown and introduce me to sambuca.

Here are a few highlights from the trip:

With MIT colleagues in San Donato.

With MIT colleagues in San Donato

Duomo di Milano. Right at the center of Milan, the Duomo is the largest cathedral in Italy and far too ornate to comprehend.

Duomo di Milano. Right at the center of Milan, the Duomo is the largest cathedral in Italy and far too ornate to comprehend.

At the Duomo with Vladimir and Pat Doyle.  (Selfie photo credit: Vladimir Bulović)

At the Duomo with Vladimir and Pat Doyle. (Selfie photo credit: Vladimir Bulović)

Papa Francesco visited Torino while I was there. Unfortunately I missed his call...

Papa Francesco visited Torino while I was there. Unfortunately I missed his call…

Running at the Politecnico di Torino, Italy’s oldest technical university.

Oddly enough, Turin is the home of one of the biggest collections of Egyptian artifacts in the world. Here's the Gallery of the Kings at the Egizio Museo di Torino.

Oddly enough, Turin is the home of one of the biggest collections of Egyptian artifacts in the world. Here’s the Gallery of the Kings at the Egizio Museo di Torino.

Focaccia at Perino Vesco. Don't miss this bakery in Torino.

Focaccia at Perino Vesco. Don’t miss this bakery in Torino.

Taking a ride in the Turin Eye, the world's largest tethered hot-air balloon.

Taking a ride in the Turin Eye, the world’s largest tethered hot-air balloon.

TIL 1 kilogram of apricots = ~25 apricots. Clearly I didn't know what a kilogram was. I wanted a snack; I got a stomachache.

TIL 1 kilogram of apricots = ~25 apricots. Clearly I didn’t know what a kilogram was. I wanted a snack; I got a stomachache.

Sardinian cuisine with Francesca and friends, captured in full Polaroid glory.

Sardinian cuisine with Francesca and friends, captured in full Polaroid glory.

Politecnico di Milano, the largest technical university in Italy and sworn enemy of its Torino counterpart.

Politecnico di Milano, the largest technical university in Italy and sworn enemy of its Torino counterpart.

Navigli: Ancient canals and home of Milanese nightlife.

Navigli: Ancient canals and home of Milanese nightlife.

The Brera Gallery is a very cool art museum in the heart of Milan.

The Brera Gallery is a very cool art museum in the heart of Milan.

The Kiss (Hayez 1859)

The Kiss (Hayez 1859). So Italian.

Kicking it at the World Expo. The theme was "Feeding the Planet, Energy for Life", aka FOOD!

Kicking it at the World Expo. The theme was “Feeding the Planet, Energy for Life”, aka FOOD!

Main street of the Expo.

Main street of the Expo.



20,000 LEDs form a room-sized floor display in the China pavilion.

20,000 LEDs form a room-sized floor display in the China pavilion.

With Francesca at the China pavilion.

With Francesca at the China pavilion.

Nutella restaurant? Count me in.

Nutella restaurant? Count me in.

Vertical farm at the Israel pavilion.

Vertical farm at the Israel pavilion.

Aquaponics = Aquaculture (raising fish in tanks) + Hydroponics (growing plants in water). The fish poop, bacteria break down the poop into nitrates, and the plants use the nitrates as fertilizer. Unfortunately you still have to feed the fish.

Aquaponics = Aquaculture (raising fish in tanks) + Hydroponics (growing plants in water). The fish poop, bacteria break down the poop into nitrates, and the plants use the nitrates as fertilizer. Unfortunately you still have to feed the fish.

What is this thing, and why is it here?

What is this thing, and why is it here?

The UK beehive pavilion. The pavilion is connected to an actual beehive in Nottingham: In the pavilion, speakers and LEDs generate noise to reflect the real-time activity of bees in the actual hive.

The UK beehive pavilion. The pavilion is connected to an actual beehive in Nottingham: In the pavilion, speakers and LEDs generate noise to reflect the real-time activity of bees in the actual hive.

Good ole USA.

Good ole USA.

Supermarket of the Future: This was very cool. It's an operational supermarket with a bunch of high-tech stuff. Point at food and the screens above show you the nutritional information, price, etc. Robot arms pick up and package fruit. And after you check out, you get to carry your grocery bags around the Expo all day. Yay!

Supermarket of the Future: This was very cool. It’s an operational supermarket with a bunch of high-tech stuff. Point at food and the screens above show you the nutritional information, price, etc. Robot arms pick up and package fruit. And after you check out, you get to carry your grocery bags around the Expo all day. Yay!

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MIT hosts unprecedented debate on fossil fuel divestment

MIT hackers transformed Building 18 into a 105-pixel display on the morning of MIT's debate of fossil-fuel divestment

MIT hackers transformed Building 18 into a 105-pixel display on the morning of MIT’s campus-wide debate on fossil-fuel divestment

Last Thursday, MIT hosted a fantastic debate on fossil fuel divestment and other university actions to fight climate change. If you’ve ever wondered why your institution should or should not divest, I highly recommend taking a look. You can watch the full debate online here (1 hour, 35 minutes).

If you don’t have an hour to spare, you can also read more about the event here:

  • Bloomberg: “Harvard Dismisses Climate Change Protesters While MIT Negotiates With Them”
  • MIT News: “MIT hosts debate on pros and cons of fossil-fuel divestment”
  • Scientific American: “M.I.T. Debates Whether to Drop Fossil-Fuel Investments”


For fossil fuel divestment:
Naomi Oreskes, Professor of History of Science at Harvard University
Don Gould, Trustee Pitzer College & CIO Gould Asset Management
John Sterman, Professor, MIT Sloan School of Management

Against fossil fuel divestment:
Brad Hager, Professor, Director of the MIT Earth Resources Laboratory
Frank Wolak, Professor of Economics, Stanford University
Timothy Smith, Director of ESG Engagement, Walden Asset Management

Six prominent climate-change figures debate fossil-fuel divestment at MIT

Six prominent climate-change figures debate fossil-fuel divestment at MIT

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What is the future of solar energy? (Quora)

Answer by Joel Jean:

The future of solar?

Let’s start with the future of the world.

To properly frame any discussion about the future of any kind of energy, we need to keep a few facts in mind:

(1)  Climate change is a real and present threat to the future of human life and all other life on Earth. Suppose we want to minimize our (children’s) risk of encountering the very worst impacts of climate change. That translates to reducing global greenhouse gas (GHG) emissions ~80% by 2050. Since ~60% of global emissions stem from energy use, we need to deploy low-carbon energy technologies at massive scale, starting yesterday.

***Details: Below is a plot of typical ranges of lifecycle (“cradle to grave”) emissions (or carbon intensity) of different energy technologies (units: grams of CO2-equivalent per kilowatt-hour (kWh) of electric output). The green dashed line is a projection of the average U.S. carbon intensity required to cut emissions by 80% (from 1990 levels) by 2050 and keep global warming below 2ºC. [1]. Wind and concentrating solar power (CSP) are by far the lowest [2]. Geothermal and solar photovoltaics (PV) are comparable. Hydro and nuclear are higher but in some cases still within range of the 2050 target [2]. Natural gas [3], coal [3], and even coal with carbon capture and storage (CCS) [2] are far above the acceptable limit.

(2) Solar is by far the largest energy resource available on Earth—renewable or otherwise. All other energy sources—aside from nuclear, geothermal, and tidal—come from sunlight. Fossil fuels are just solar power integrated over millions of years using dinosaurs (and other carbon-based life forms) as batteries. Wind and wave power is merely solar power absorbed unevenly across the Earth’s surface, leading to thermal gradients and mass flow. Among low-carbon energy sources, only solar, wind, and possibly nuclear can reach the terawatt (TW)-scale deployment needed to satisfy ever-growing global energy demand (currently ~17 TW average).

(3) Solar photovoltaics is growing fast—faster than any other energy technology. Cumulative installed PV capacity worldwide has doubled every two years (43% CAGR) since the year 2000, reaching ~200 gigawatts-peak (GWp) in 2014. This Moore’s Law-like growth shows no sign of slowing, though slow it must, as naive extrapolation leads us to some untenable conclusions: If PV capacity were to keep growing at the current rate, solar panels would satisfy all world electricity demand within a decade, cover the Earth by 2050, and form a Dyson sphere around the sun just after 2100.

Just for fun, here’s the naive extrapolation:

That said, solar PV accounted for only ~1% of our total electricity consumption last year, so there’s clearly a lot of headroom left.

OK. So now we know a few things: Climate change is happening, we need lots of low-carbon energy to stop it, solar is one of our only practical options, and solar PV is growing faster than anyone ever imagined.

But how do we turn sunlight into useful energy? What’s the future of PV? And are there other non-PV solar technologies in the R&D pipeline?

Let’s talk about the technology.

Solar Photovoltaics (PV)
Solar photovoltaics (aka “solar cells”) are by far the leading solar technology in terms of total deployment*. PV is quite nice: It’s truly modular (a single PV module is no less efficient than a huge array), it operates silently and at low temperatures, and it doesn’t require much maintenance over its 25+ year lifetime.

*Aside from solar heaters, which are used widely in China for heating domestic water and in the U.S. for keeping swimming pools warm. Solar heating can’t be compared directly with PV since its output is heat [GW-thermal] rather than electricity [GW-electric].

[Solar-Powered Camel Clinics Carry Medicine Across the Desert]

We typically name PV technologies by the material (or material class) used to absorb light: crystalline silicon (c-Si), gallium arsenide (GaAs), hydrogenated amorphous silicon (a-Si:H), cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), copper zinc tin sulfide (CZTS), organics, perovskites, or colloidal quantum dots (QDs), to name a few.

It’s convenient to think about these technologies in terms of material complexity, which corresponds roughly to the number of atoms in a unit cell, molecule, or other repeating unit of a material [4,5]. Material complexity is related to the degree of disorder at the nanoscale. For current PV technologies, higher material complexity often translates to lower technological maturity, materials use, processing temperatures, and processing complexity. These traits often open up new applications by enabling novel technical attributes, such as visible transparency, flexibility, and new form factors.

With PV technologies, it’s really hard to predict what will be the long-term winner.

Crystalline silicon (c-Si) is king today, with ~90% of the global PV market, and I believe it will continue to dominate for at least the next decade. Silicon PV is abundant, efficient, reliable, and proven, but it absorbs light poorly. That drawback results in thick, heavy, inflexible solar cells and modules with relatively high manufacturing costs. For silicon, there’s not much room to grow in terms of cell efficiency (25% current lab record), although production modules continue to improve: Typical modules are 16-21% efficient, with multicrystalline (mc-Si) technologies at the low end and single-crystalline (sc-Si) technologies at the high end.

Today’s commercial thin-film (TF) PV technologies, including CdTe, a-Si:H, and CIGS, overcome some of the challenges of c-Si—they use much less material and can be made at relatively low cost with high efficiency. CdTe dominates the thin-film market with simple manufacturing and high efficiency (21% cell record, production modules up to ~15%) but has major intrinsic scaling issues: Tellurium is about 4x less abundant than gold in the Earth’s crust, and it’s hard to extract from copper ores. Amorphous silicon is abundant, cheap, and flexible, but its maximum efficiency (13.4% current cell record) is likely too low to compete with crystalline silicon. CIGS is efficient (21.7% cell record, modules up to ~15%) but tough to make reliably, and it also runs into materials scaling issues with indium, gallium, and selenium.

In the PV R&D community, we pursue emerging thin-film PV technologies, such as perovskites and quantum dots, for 2 primary reasons: (1) They may someday be able to reach a lower cost per watt than silicon and current thin films due to simpler manufacturing and reduced materials use, and (2) They offer new functionality, including transparency, flexibility, and extremely light weight, and may open up new applications for PV.

Examples of emerging thin films include CZTS, organic PV, dye-sensitized solar cells (DSSCs), perovskites—which have largely swallowed the organic and dye-sensitized PV R&D communities—and colloidal quantum dot PV (QDPV). Perovskites are extremely promising, with impressive material characteristics and cell efficiencies improving at an unprecedented rate (up to over 20% in ~3 years). But we shouldn’t get too excited yet—there’s still a lot of work to be done, in particular on lifetime, air and water stability, and new cell designs. Although still relatively inefficient (~9% cell record), QD solar cells are also improving fast and can be processed entirely at room temperature from solution, which may someday lead the way to the fabled “solar paint” (which, contrary to popular belief, does not yet exist in any practical form).

See the bottom of this post for several FAQs about solar PV.

Concentrating Solar Power (CSP), aka Solar Thermal
CSP uses mirrors or lenses to concentrate sunlight onto a tank of molten salt or other working fluid, which is then used to boil water and drive a steam turbine. CSP systems have been used for decades, but they only work effectively in places with high direct radiation*—such as the southwestern U.S., southern Europe, northern Africa, and other locations near the equator.

*The MIT Future of Solar study recently analyzed the cost of CSP in Worcester, Massachusetts, and… nope, not a chance.

CSP is not modular like PV—high temperatures require many mirrors over large areas, and turbines are much more efficient at large scale—so unfortunately you won’t have a solar thermal generator on your roof anytime soon. Here’s a picture of the new Ivanpah CSP plant in the Mojave desert (opened Feb. 2014), with over 340,000 mirrors:


Global CSP deployment today is lower than PV deployment by about 2 orders of magnitude. As for the future, CSP will become more and more important as penetration of solar and wind increases, because it can potentially overcome the natural intermittency of those resources (discussed further below) using built-in thermal energy storage (on the time scale of 4-8 hours).

Solar Fuels
Sunlight can catalyze chemical reactions that use water and CO2 to produce liquid or gaseous fuels (e.g., hydrogen, methane, various alcohols and hydrocarbons). These “solar fuels” have a unique role in a future low-carbon energy economy, since they could help decarbonize transportation—especially by air and sea, where electric-powered transport may be impractical. Solar fuels could also become a key energy storage technology for counteracting solar intermittency.

All that said, solar-to-fuels technology is far from proven—my MIT colleague Bob Jaffe would say that there are many “tooth fairies'” worth of fanciful technological advances that still need to be made to get solar fuels to market at competitive cost.

Technologically, the future of solar energy looks bright.

So what’s stopping solar from taking off? And what might limit it in the future?

Well, there are a few things that might be worth thinking about: cost, intermittency, and scaling issues (i.e., materials and land use). Let’s focus on PV for now.

Solar PV is getting cheaper by the month. Average system costs in the U.S. are now below $2/W for utility scale (>1 MW) systems and just over $3/W for residential (usu. <10 kW) systems [4]. And that doesn’t include subsidies (e.g., 30% federal investment tax credit (ITC), accelerated depreciation, various state renewable portfolio standards (RPSs), and net metering in some places), which in a competitive market can reduce the effective price for consumers. In regions with high direct sunlight, such as southern California, CSP is cost-competitive with PV [4].

So what’s the problem?

System cost [$/W] isn’t a complete metric. Solar energy technologies will only take off if they can produce and deliver electricity more cheaply than alternatives.

The usual way of comparing the cost of generating electricity with different technologies is the levelized cost of energy (LCOE), in units of $/kWh (usu. “cents per kilowatt-hour”) or $/MWh.

The LCOE of a power plant includes upfront capital costs, operation and maintenance costs (including fuel), subsidies, an assumed discount rate, and the total electricity produced over the plant lifetime. Typical LCOEs are $0.06-0.08/kWh for coal, natural gas, and hydro and ~$0.10/kWh for nuclear.

Most LCOE estimates for unsubsidized solar PV today range from ~$0.10/kWh to $0.40/kWh, depending on the location (i.e., amount of sunlight) and type of system (large utility-scale systems are cheapest, small residential systems most expensive). In places like Hawaii, where fuels are hard to come by and hence expensive*, the LCOE of conventional fossil fuel generation is much higher and more volatile, and solar is much more attractive. But LCOE alone isn’t the whole story either.

*Because it lacks local fossil fuel resources, Hawaii uses imported oil to generate most of its electricity, unlike the rest of the U.S. As a result, electricity prices in Hawaii are highly correlated with global oil prices.

Even in places where solar costs more than coal (and gas and nuclear)—and even when subsidies are included—you can sometimes still save money by putting solar panels on your roof. Why?

Well, your solar electrons aren’t competing directly with the grid’s electrons on cost. Electrons from a coal plant (e.g.) are purchased by a utility and transmitted to your house on the grid. The utility can’t give you the electrons at the same price they were bought for: It has to make a profit, and it has to amortize the costs of building and maintaining the grid. So the actual retail price of grid electricity—what you see on your electricity bill—is substantially higher than the underlying LCOE. Under current regulations (in many states), your rooftop solar electrons can compete with grid electrons at the retail price, and if your PV system makes more electrons than you use, you can sell them back to the utility at the retail price—a practice called net metering. It’s basically a subsidy for solar and a great deal for you, although utilities don’t like it because you’re not paying your share of the grid upkeep. Future policies will likely close that loophole by forcing homeowners with PV systems to pay some fixed cost for grid use.

Fortunately, it seems likely that the cost (LCOE) of solar will continue decreasing steadily, which means that unsubsidized solar will eventually be cheaper than fossil generation in many places—especially if a price is placed on greenhouse gas emissions to account for their negative environmental externalities.

For solar PV in particular, it’s important to note that the total system cost is no longer dominated by the solar panel itself. Everything else that goes into a PV system—inverters, transformers, wiring, racking, installation labor, customer acquisition, permitting, taxes, financing, business overhead—add up to well over half of the total cost of solar PV in the U.S. today. We refer to these non-module costs as balance-of-system (BOS) costs, and in many ways, they’re harder to reduce than module costs. To realize a solar-powered future, we need to innovate and reduce BOS costs.

In most places on Earth, sunlight isn’t always available. Some of the variations in available solar energy are predictable or deterministic (e.g., diurnal and seasonal cycles and local climate), while others are unpredictable or stochastic (e.g., cloud cover and weather).

When solar is deployed at large scale (several percent of total electricity generation) on a given grid, electricity markets will likely change significantly. After a solar PV system is installed, it costs almost nothing to operate. Zero-variable-cost generation means that solar energy will be used whenever it’s available (i.e., when the sun is shining). PV will thus replace fossil-fueled generators (i.e., coal and gas) with the highest variable costs, reducing marginal electricity prices.

But the impact on actual market prices depends strongly on the generation mix—i.e., how much coal, gas, nuclear, wind, etc. is deployed on the grid: Solar tends to stop producing when the sun goes down—and when the clouds come out—so other generators are forced to ramp up (cycle) more rapidly and more often, increasing wear-and-tear and hence their operating costs.

In many places, adding solar PV without energy storage doesn’t substantially reduce the net load that must be supplied by other technologies (total demand – solar generation). It simply shifts the time of peak load slightly later into the evening, when the sun goes down and everyone goes home after work and starts watching TV, cooking, reading Quora, coding, or otherwise consuming electricity (this is the origin of the famous CAISO duck chart).

A few complementary technologies would ease the pain of intermittency. In decreasing order of current technological and economic feasibility: Enhanced grid infrastructure (e.g., improved long-distance transmission, demand response, and other smart grid concepts) could adjust demand to meet varying solar supply, or allow geographical averaging to smooth out minute-to-minute variations due to clouds. Grid-scale energy storage (e.g., pumped hydro, compressed air, or big batteries) could store energy during the day and discharge it at night. And solar fuels could someday make solar energy truly dispatchable.

Scaling: Materials and land use
The land use issue is actually not that big a deal. To satisfy all U.S. electricity demand with solar PV at average solar insolation levels, we would need on the order of 50,000 square kilometers [4]—which sounds like a lot, until you find out that we currently dedicate ~100,000 square kilometers to producing corn ethanol satisfying only ~10% of U.S. gasoline demand.

Materials use is a bigger concern: Covering thousands of square kilometers with PV will require huge amounts of raw materials, from the elements used in solar cells to supporting commodity materials, such as steel, glass, and concrete [4,5].

Our analysis suggests that most commodities will not be major obstacles to scaling, except perhaps the flat glass used to cover today’s c-Si PV modules [4,5]. For glass, aluminum, and copper, the amount of material required to satisfy 100% of 2050 world electricity demand exceeds 6 years of current global production, which indicates that PV might eventually become a major driver for those commodity markets.

Critical elements will be limiting for some technologies:

  • For c-Si, silicon is not an issue, but silver conductors will need to be replaced by copper.
  • For CdTe, Te is a huge obstacle to scaling: At current rates of global Te production, we would need 1500 years to extract all the Te required to satisfy 100% of 2050 world electricity demand with CdTe PV [4,5].
  • Emerging thin films are generally more scalable than current commercial technologies, since they mostly use small amounts of abundant, widely-produced elements: For example, to satisfy 100% of 2050 demand with PbS QDPV, we only need the equivalent of 23 days of global lead production and 7 hours of current sulfur production [4,5].

So what is the future of solar?


The main takeaway is that there will be more solar than you think. To spare future generations from the worst impacts of climate change, we’re going to need a lot more solar (and wind and nuclear) generation capacity—10-100 times what’s currently deployed. Today’s technologies (mostly crystalline silicon PV) can—and likely will—scale up to multiple terawatts of capacity worldwide by 2030 without any major technological advances.

The main obstacle is cost: Global PV growth thus far has largely been driven by federal and local subsidies. That said, PV is already cost-competitive with fossil fuels in some places, and system costs (and prices) continue to decline. And even though current technologies will likely plateau at some minimum sustainable cost floor, it’s clear that there are many new and exciting solar technologies in the pipeline, with many new and exciting applications to come. I can’t wait.


[1] J. E. Trancik and D. Cross-Call, Environ. Sci. Technol., 2013, 47, 6673-6680.
[2] M. Z. Jacobson, Energy Environ. Sci., 2009, 2, 148-173.
[3] U.S. Energy Information Agency (EIA), 2014.
[4] MIT Future of Solar Energy Study, 2015, in preparation.
[5] J. Jean et al., 2015, submitted.

*Just to be clear, the views expressed here are my own. My opinions are informed by my involvement in the MIT Future of Solar Energy Study but don’t necessarily reflect the final conclusions of that study. I encourage you to read the report when it’s released later this year (and others) and come to your own conclusions about the future of solar.

**Feel free to use the figures in this answer for educational purposes. Figures without citations are my own, and proper attribution is appreciated.

Solar PV FAQs

Does a solar panel produce more energy than it takes to manufacture it? In other words, is the energy payback time (EPBT) shorter than the lifetime of the panel?

YES! A typical silicon PV module today produces as much energy as it took to manufacture it in less than 2 years (<1 year for CdTe), and continues to operate with minimal efficiency loss for at least 25 years.

I hear solar cells can only convert 15% (or 20%) of incoming sunlight into electricity. Why are solar cells so inefficient? I mean, my body can convert a Big Mac into useful energy at 25% efficiency. Why can’t you scientists do better?

Physics is tough to beat. Thermodynamic limits (see Shockley-Queisser limit) cap the ultimate efficiency of typical solar cells based on a single material at ~31%. Using multiple materials—as in multijunction or tandem cells—boosts the theoretical maximum efficiency (e.g., to ~49% for 3 junctions). Some “exotic” approaches (e.g., carrier multiplication, hot-carrier collection, and intermediate band cells) can theoretically bypass the Shockley-Queisser limit, but none has yet achieved practical efficiency gains.

But in the end, we don’t really care about efficiency anyway; we care about the cost of energy [$/kWh]. Photosynthesis is on the order of 1% efficient at converting sunlight into chemical energy, yet the U.S. still tries to grow corn to make ethanol. That said, it’s worthwhile to work on improving the efficiency of solar cells because higher efficiencies can decrease module and system costs.

Why are production PV modules so much less efficient than record cells in the lab?

Intrinsic scaling losses: Scaling from small cells (~1 square centimeter) to large modules with multiple interconnected cells (~100 square centimeters) incurs physical scaling losses. Electrons must travel farther, increasing resistive losses. Shadowing from electrodes reduces the available light. Longer wires in modules dissipate more power, while spacing between cells reduces the module active area. The output of a module is often limited by its worst-performing cell.

Extrinsic manufacturing losses: While researchers typically target high efficiencies without much regard to cost, manufacturers may sacrifice efficiency to reduce cost, improve yield, and increase throughput. Fabrication techniques that produce high efficiencies in the lab may not scale to large areas. High-quality materials used in research labs may be too expensive for high-volume manufacturing.

Do solar cells work when it’s cloudy?

Yep, although somewhat less efficiently. Clouds and atmospheric particulates turn direct sunlight (what you see when you look right at the sun) into diffuse light (what you see when you look anywhere else in the sky, or at the ground). A solar cell doesn’t really care where photons come from—whether scattered from a cloud or transmitted straight through the atmosphere. Once a photon of a given color is inside a solar cell, it’s converted into an excited electron with the same probability as any other photon of that color.

But a solar cell DOES care about how many photons are incident on it—that is, the light intensity. Higher intensities generally give higher efficiencies. Because some direct sunlight is scattered back into space by clouds—and because more light is lost to reflection at the front surface of the panel at wide angles of incidence—the light intensity that a solar cell sees is lower on overcast days than on clear days, leading to lower efficiency and lower power output.

On a related note, concentrating solar technologies (CSP and CPV) don’t work at all when it’s cloudy: You can’t concentrate light that’s coming from all directions.

Are solar cells toxic?

Yep—but only if you eat too many of them. In all seriousness, silicon solar cells are about as benign as any other piece of technology that you probably wouldn’t eat. Cadmium telluride is a bit worse—cadmium is toxic—but the amount of Cd inside a typical solar cell is quite small. The CdTe layer used in cells today is about 1/50th the thickness of a sheet of paper, or roughly 2 µm, and it’s sealed up tight between two thick glass sheets. As for emerging technologies, you’d have to eat a lot (!) of solar cells to get lead poisoning from lead sulfide quantum dot PV or methylammonium lead halide perovskite PV.

When solar electricity becomes cheaper than current grid electricity (“grid parity”), will it completely displace all other generation technologies?

Nope. We’ll probably still need other types of generation—certainly at night, and likely during northern winters as well. Like any other free market, electricity markets generally follow the laws of supply and demand. Increasing supply of zero-marginal-cost electricity from solar PV depresses electricity prices at exactly the hours when solar is available (i.e., when the sun is shining). For any given electricity grid and cost of PV, there will likely be a natural break-even point for solar penetration—above that level of deployment, additional solar generation will no longer be profitable, and further investment in solar is unlikely. The story might change if grid-scale electricity storage gets way, way cheaper or if CSP (with built-in thermal storage) is deployed widely.

What is the future of solar energy?

Meet the MITEI External Advisory Board

With my advisor Vladimir and former U.S. Secretary of State/Treasury/Everything George Shultz

With my advisor Vladimir and former U.S. Secretary of State/Treasury/everything George Shultz

I gave a talk last week that made me a bit nervous.

The audience? The External Advisory Board of the MIT Energy Initiative (MITEI). The EAB deeply influences MIT’s energy research direction and represents a large fraction of research funding on campus. It also happens to have on it some of the more distinguished people and fancy titles in the world of energy and climate, including former members of Congress, ex-Secretaries of State/Energy/etc., heads of national organizations, VCs, oil and gas executives, and a couple Nobel laureates for good measure.

Yep. I was a bit nervous.

This was an unusual opportunity. I was invited to give a 15-minute talk on solar photovoltaic technology as a member of the MIT Future of Solar Energy Study panel. The Solar Study is the latest of a series of MITEI-sponsored “Future of ____” reports meant to inform the public and guide policymakers in D.C. on the current status and future trajectory of leading energy technologies. The report won’t be ready until the end of the year (fingers crossed), but naturally the EAB wanted to hear all the juicy details firsthand.

Given the audience, I was expecting to be regularly interrupted and thoroughly questioned, especially by our friends from Shell and Saudi Aramco. I was mistaken. My talk went smoothly, no one interrupted, only a couple people fell asleep, and several questions during the panel sparked interesting conversations. No sweat.

That evening, the EAB and all of the speakers ended up at President Reif’s house for dinner. A few highlights: (1) Kerry Emanuel gave a talk about the science of climate change. (2) My advisor Vladimir introduced me to George Shultz, former U.S. Secretary of State (during the Reagan administration) and current Hoover Institution fellow at Stanford—93 years old and still going strong. (3) Dinner was delicious.

What a day.

***After the panel, I was chatting with one of the board members, Frances Beinecke (NRDC president), and found out that she marched in the People’s Climate March! Awesome.

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The People’s Climate March

climate march

“Look at all these people here because they care about your future.”
– Parent to child while observing the People’s Climate March, overheard by Geoffrey Supran

This past Sunday, together with 70 others from MIT, the Incredible Hulk, and 400,000 more from across the U.S. and around the world, I marched for climate change in New York City.

I marched because if our generation doesn’t deal with climate change, the next generation may not be able to.
I marched because I believe a price on carbon is the most efficient road to a livable future climate.
I marched because the world leaders convening in New York on Tuesday needed to see us march.
I marched because those who have the privilege to know have the duty to act.
I marched because otherwise my children would someday ask why I didn’t.
I marched because I stand with science.
I marched because there is still hope.
I marched.

I marched because it was the People’s Climate March—the biggest climate-change demonstration in history, a statement of the people, by the people, for the people.

I had no idea what to expect. I’ve never been an activist before. All I knew when I boarded a bus from the Alewife T station outside Boston on Saturday morning was that 500 buses carrying 25,000 people from all over the country would converge on New York City in the next 24 hours. Aside from 8 MIT students and staff, my bus, hosted by 350MA, was filled with mostly older folks from Cambridge, Somerville, Lexington, Concord, and a few other sleepy suburbs of Boston. Maybe it would be a quiet weekend.

At 10AM on Sunday morning, we assembled just west of Central Park between 59th and 86th St. for the start of the march. The MIT contingent crowded into the student section at 69th St. and Central Park West. More and more people trickled in. By 11AM, hundreds of thousands of people overflowed the 30-block-long assembly area—so many that after the march started at 11:30AM, those near the back didn’t start moving until 2 hours later. It was hot, sticky, overcast, altogether unpleasant. But no one complained. The day was about climate, not weather. A moment of silence at 12:58PM honored those already affected by climate change; a tidal wave of noise two minutes later sounded the alarm to the world. So the march began.

For context, I’ve been working with Fossil Free MIT (FFMIT) for over a year now, pushing MIT to take strong action on climate change, in particular by divesting its $12.4B endowment from companies with major reserves of coal, oil, and natural gas. FFMIT is part of a growing international divestment movement among individuals, companies, and institutions both public and private. It just makes sense. When you study the global impacts of climate change, when you look at the urgency of near-term action and the looming carbon bubble, when you find out that political donations are blocking climate legislation, when you ask the mirror, “What can I do about climate change?”, divestment is the logical answer. Fossil-fuel divestment is all about leveraging institutional clout to achieve a better future. I can’t think of a stronger action that students can take today.

On Sunday, I marched with 50,000 students from MIT, Stanford, Harvard, BU, BC, Berkeley, Tufts, Yale, Duke, UNC, Tulane, and hundred of other schools; with Al Gore and Leonardo DiCaprio and the U.N. Secretary-General Ban-Ki Moon; with men and women and children from every state and every socioeconomic status. All day the chants echoed through the streets of New York:

“What do we want? Climate justice!
When do we want it? NOW!”

“Show me what democracy looks like.
THIS is what democracy looks like!”

“Hey hey—ho ho—fossil fuels have got to go!”

For 4 hours we marched, south and east around Central Park, past Trump Tower and the Bank of America Tower and the Fox News building, through Times Square and all the way to 11th Ave. We marched with banners, signs, posters, body paint. Scientists marched with a giant chalkboard showing rising CO2 levels and rising global temperatures. Students marched with a 30-foot inflatable globe. Parents marched with kids in Lorax costumes. Rappers and musicians marched with boom-boxes, with megaphones, with loud voices and louder conviction. All marched with pride.

But climate change wasn’t the only issue at hand. The marchers represented causes ranging from climate change to clean water, social justice to global health, labor unions to indigenous communities, veganism to socialism. The message to the U.N. and to world leaders would have been stronger had everyone been asking for the same thing. My vote would be for a price on carbon, widely accepted as the most economically efficient solution to climate change. But seeing so many communities come together on one street, if not one issue, was truly an inspiration: Every advocate for every cause knew that unchecked climate change would weaken their own cause, deepen economic prejudice and injustice, and ultimately create more problems for more people. Each one knew the terrible and beautiful truth, that “to change everything, we need everyone.”

I was in a meeting a few months ago when the discussion turned to climate change, and someone made a keen observation: If we don’t solve cancer or education or just about any other problem in the world, nothing will change. Life as we know it will go on. But if we don’t solve climate change, things will change. Life as we know it will not go on. This is a fact. The most conservative thing any of us can do today is to move boldly against climate change, just like 400,000 people did last Sunday at the first-ever People’s Climate March.

A few shout-outs to…
Anastasia for being a gracious host last weekend
Patrick for a heroic organizing effort
Jenn and Alison, my banner buddies
Ploy for her climate artistry and for being featured all over the national news
Geoffrey for being appointed to the MIT Climate Change Conversation Committee
All my fellow MIT marchers who made the PCM so damn inspiring

2014-09-21 23.06.48
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Googling grad students

I googled “grad student” along with a number of different search terms. Here are the results, scientifically speaking:

Google grad student

The lesson here isn’t that you shouldn’t go to grad school… Just make sure you always use a log scale.

Edit (20130903): This plot was featured on PhD Comics!

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The Things They Carried: All aboard the Carnival Paradise

Last weekend’s Caribbean cruise with my brother and friends left me with some lessons in packing, a boatload (ha ha) of memories, and a few supremely entertaining photos.

For Future Me and anyone else planning and packing for a cruise, here’s a list of items you won’t remember to bring but might turn out to be critical (by some hedonistic criteria):

  • Water and drinks – Only lemonade and iced tea are provided. If you want anything to drink besides ship tap water without paying exorbitant fees, bring it with you.
  • Alcohol – Ditto. Carnival allows each person to bring one bottle of wine onto the ship; everything else is confiscated UNLESS you’re sneaky. Contact Robin Tan for tips on how to be sneaky.
  • Cash – Always good to have. Especially if you like to gamble. (Please gamble responsibly.)
  • Point-and-shoot camera – Cell phones don’t work on a ship for ordinary people on ordinary budgets, so there’s no reason to carry one around. Bring a dedicated camera for higher-quality memories.
  • Walkie-talkies – How else are you going to coordinate that 19th group outing to the 24-hour pizza buffet on Lido deck?
  • Games – There’s always time for games. Too much time.
  • Speakers – Just in case you want to listen to something besides that old dude singing karaoke.
  • Gym clothes – You’ll need to work out hard to power through that fourth appetizer at dinner.
On to the photos: The best of the bunch are simply too awesome for public consumption and will be left to the imagination out of respect for the subjects’ careers and dignity, but here are a few solid runners-up.
This guy is cooler than you.

This guy is cooler than you.

This guy just thinks he is.

This guy just thinks he is.

Sometimes you lose money at the casino. Sometimes Carlina wins $1000 in cold, hard cash.

Sometimes you lose money at the casino. Sometimes Carlina wins $1000 in cold, hard cash.

Beka and Robin only won $200. Weak.

Beka and Robin only won $200. Weak.

Offloading ourselves in Cozumel.

Offloading ourselves in Cozumel.

Candid shot.

Candid shot.

Looking for a sunken ship during the Amazing Race in Cozumel.

Looking for a sunken ship during the Amazing Race in Cozumel.

We win!

We win!

Gold medals!

Gold medals!

At the beach.

At the beach.

The first in a series of pool escapades.

The first in a series of pool escapades.

Human totem pole, AKA Double Chicken.

Human totem pole, AKA Double Chicken.

Human pyramid. Why are the girls always on top?

Human pyramid. Why are the girls always on top?



Last hurrah in Cozumel.

Last hurrah in Cozumel.

Edible happiness at breakfast.

Edible happiness at breakfast.

Birthdays happen at sea too. Happy 24th and 25th, Carlina and Justin!

Birthdays happen at sea too. Happy 24th and 25th, Carlina and Justin!

The boys.

The boys.

The Duke girls.

The Duke girls.

On a post-cruise diet in Tampa.

On a post-cruise diet in Tampa. Thanks John and Robin!

Team pic: Carlina, me, Robin, Neal, Justin, Dave, Beka, Nancy. Thanks for a ridiculously awesome weekend!

Team pic: Carlina, me, Robin, Neal, Justin, Dave, Beka, and Nancy. Thanks for a ridiculously awesome weekend!

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It’s just the future of the world. No big deal.

Never would I have thought that something as distant and mundane as the climate could upset me so.

But I’ve been pondering climate change more and more with every passing week, and more and more it’s becoming clear that we (humans) have two options if we want to avoid the icky consequences of climate change:

  1. Stop burning fossil fuels for energy.
  2. Find a new planet to live on.

Tongue-in-cheekiness aside, I truly do believe in Option 2—space exploration—as a vital long-term goal for humanity. Unfortunately, the time for climate action is limited, and new worlds aren’t built in a day. So today I thought it would be a good idea to link climate change with my own interests and background in energy, to summarize and share some of the unadulterated facts. I hope to inspire you to take a moment to think, to personalize and internalize the numbers, to dedicate your own prodigious intellectual resources to finding a solution to our shared little problem.

I draw numbers and inspiration (and a few direct quotes) from a 2009 review paper by Stanford’s Mark Jacobson, whose work I highly recommend to anyone interested in energy and the environment.

Climate Change and Air Pollution

Key point: Climate change and air pollution have a host of deleterious effects on human and environmental health.

Climate change (or global warming) is caused by an increase in the greenhouse effect induced by the following human inputs into the atmosphere (from most to least significant):
  • CO2
  • Fossil and biofuel soot
  • Methane
  • Halocarbons
  • Tropospheric ozone
  • Nitrous oxide

So what? Well, here’s my own little climate-change mnemonic:

The effects of climate change are FAST.

  • Food and Freshwater
    • Shifts viable locations for agriculture
    • Shifts the timing and magnitude of freshwater availability
  • Animals and plants, Acidity
    • Alters natural habitats substantially, causing extinction of plant and animal species
    • Increases the acidity of the ocean (dissolved CO2 forms carbonic acid, H2CO3) – The pH of the world’s oceans has dropped by ~0.1 since pre-industrial levels
  • Storms, Sea levels, and Sickness
    • Makes tropical storms more severe
    • Shrinks glaciers, snow pack, and ice levels worldwide, causing sea levels to rise
    • Increases the prevalence or geographic spread of various diseases (e.g., West Nile)
  • Temperature
    • Increases global temperatures and accompanying heat-related health risks

Air pollution is the 6th leading cause of death worldwide; it has been implicated by the WHO in 2.4 million deaths worldwide each year. Indoor and outdoor pollutants can increase ER visits, reduce worker productivity, and induce all of the following health conditions: asthma, respiratory illness, cancer, and cardiovascular disease.

Slightly troubling note: Human-created aerosol particles in the atmosphere—sulfates, nitrates, chlorides, ammonium, potassium, carbon, H2O—mask ~50% of the current global warming potential; if we reduce air pollution, much of that warming potential will be unveiled.

Energy Production and Use

Key point: Both climate change and air pollution are fundamentally linked to energy production and use; both arise from the burning of solid, liquid, and gaseous fuels. Solving these problems will thus require fundamental changes to the energy sector.

A few important numbers on energy and emissions:

  • Global energy production = 133 PWh/yr = 15.2 TW (2005)
  • Global electricity production = 18.2 PWh/yr = 2.1 TW (2005)
  • 25% of total US CO2 emissions are directly exhausted from vehicles on the road; another 8% are due to production and transport of fuel. Operation of fossil-fuel-burning vehicles thus accounts for over 33% of total US CO2 emissions, which we can eliminate by powering vehicles with non-emitting or lower-emitting technologies.
  • In the US, each person uses an average of ~14,000 kWh of electricity per year.
When evaluating alternative energy technologies, we need to think about several key metrics and consequences:
  • Resource availability: How much raw energy is available in a given energy resource? A renewable technology that’s extremely efficient and always available may have very little impact on climate change or pollution if the available potential energy is not enough to satisfy a major fraction of total energy demand.
  • Total CO2 emissions: Emissions can be measured in grams of CO2 equivalent per kWh (CO2e/kWh): In the context of emissions (see clarification here:, this unit refers to the amount of CO2 required to produce a global warming effect equal to whatever is emitted by a particular technology, for each unit of energy produced.
    • Lifecycle emissions: How much CO2 (equivalent) is emitted during the production, operation, and disposal of a given technology for each unit of energy generated? Higher lifecycle emissions make a technology less attractive for mitigating climate change and air pollution.
      • The energy payback time is the amount of time for which a power plant must operate to produce the same amount of energy that was required to build it.
      • A longer technology lifetime amortizes the fixed CO2 emissions over a longer period of energy generation, reducing the emission per unit of energy produced.
    • Opportunity-cost emissions: If a renewable generation source takes a long time to plan and deploy, significant emissions may occur during that period; those emissions could have been eliminated if a more quickly deployable renewable source were used instead.
      • The planning-to-operation time encompasses the time to site, finance, permit, insure, construct, license, and connect the technology to the grid.
      • Such delays allow existing, higher-emitting power generation plants to operate longer, and limit our ability to respond to changing energy needs and the need for immediate action against climate change.
      • To minimize opportunity-cost emissions, we want generation sources with a long lifetime and fast planning and installation.
  • Land use and effects on wildlife: Renewable energy sources tend to require more land area than conventional power plants, due to the low energy density of renewable resources. Appropriation of land for energy generation may encroach on or damage wildlife habitats, as well as reducing the total amount of land available for alternative human use.
  • Vulnerability to disruption: Disruptions in the energy supply affect all human activities that are tied to the electrical grid. Centralized energy sources (e.g., nuclear plants) increase the risk of disruption compared to distributed sources (e.g., wind turbines, solar cells), since a single localized catastrophe can wipe out a large portion of local generation capacity.
  • Intermittency: Many renewable resources are by nature dependent on the caprices of complex and unpredictable environmental systems. Wind turbines produce energy only when and where the wind blows. Solar cells and solar thermal converters operate efficiently only when the sun shines on them directly, with no clouds, haze, or human obstructions in between. An energy technology that suffers from high intermittency may not be able to reach high penetration—i.e., to satisfy a large proportion of electricity needs—because it would be unable to fulfill electricity demand consistently. Other technologies must then compensate for the shortfall.
    • Intermittency makes grid-planning more difficult and increases the stress (from ramping) on other generation technologies.
    • The capacity factor is the average fraction of the peak rated power output of an energy source that is sustained continuously over the source lifetime.
      • The capacity factor can be calculated by dividing the total energy actually produced in a given time period by the total energy produced if the source produced 100% of its peak output for that entire period: If a 100Wp solar cell produces 50W for 6 hours a day and 0W for the remaining 18 hours, the numerator would be 50W*6h = 300Wh, the denominator would be 100W*24h = 2400Wh, and the capacity factor would be 300/2400 = 0.125.
      • High intermittency reduces the capacity factor of a given technology.
    • Several strategies may reduce the impact of intermittency:
      • Average out variations by combining generation sources from wide geographical areas. This requires a significant investment in grid infrastructure for long-distance electrical transmission.
      • Smooth out total generation by combining multiple high-intermittency renewable sources (e.g., solar, wind, wave) and using low-intermittency sources (e.g., hydro, geothermal) to provide for excess demand.
      • Use smart meters to charge and discharge electric vehicles at appropriate times to match electricity demand to supply.
      • Deploy an efficient utility-scale energy storage technology: Charge a battery with a solar panel when the sun is shining, and discharge the battery at night to charge an electric vehicle or iPhone. Such a technology does not exist today.
      • Forecast the availability of renewable sources more precisely (e.g., when the sun will shine and when the wind will blow) and ramp up other power plants to offset any excess demand.

Alternative Energy Technologies

Note on energy units: Although terawatts (TW = 1012 watts (W)) are units of power (energy per unit time), I will use it here to describe the total energy available for each renewable technology. To facilitate comparison with our global time-averaged energy production and demand (~15 TW total, of which ~2 TW is in the form of electricity), I’ve converted the total available energy (given by Jacobson in petawatt-hours (PWh) = 1015 watt-hours (Wh)) to the continuous available power (in W) by dividing by the number of hours in a year (8766 ~= 9000 ~= 104 for order-of-magnitude estimates). I think the lack of consistent and personally relevant units cripples clear discussion and understanding of energy issues (what the hell is a tonne of oil equivalent (toe) anyway?)—I like to stick with two units: watts (or kW, MW, GW, and TW) for power, and watt-hours (primarily kWh) for energy.

Solar energy: Concentrated solar power (CSP) and solar photovoltaics (PVs)
  • Essence of CSP: Concentrate sunlight to heat up a thermal transfer medium and generate steam to run a turbine.
    • Leading CSP technologies: Linear parabolic trough, power tower, parabolic dish (no thermal storage)
    • One primary benefit of CSP is the built-in storage capacity: By heating up and storing a material with sufficient heat capacity (e.g., molten nitrate salts (NaNO3, KNO3), steam, synthetic oil, graphite), we can counteract the intermittency of solar energy.
    • Total available energy: ~1000 TW = ~2/3 that of solar PVs (the area required per MW is ~1/3 larger for CSP than for OVs)
    • Capacity factor: 0.13-0.25 (without storage)
    • Lifetime: ~40 years
    • Energy payback time: 5-7 months
    • Planning-to-operation time: 2-5 years
    • Total emissions: 9-11 g CO2e/kWh
  • Essence of PVs: Use an energetically asymmetric semiconductor device to absorb light and generate electricity.
    • Leading PV technologies: Si (amorphous, polycrystalline, microcrystalline), Thin-fim (CdTe, CIGS, CZTS)
    • Solar cells can be deployed in large farms (usually 10-60 MWp, proposed up to 150 MWp) or on rooftops (90% of installed PV capacity is currently on rooftops; a future estimate for that number is 30%). Even the largest solar farms are smaller (in peak capacity) than average fossil-fuel (~600 MWp) or nuclear plants (~1 GWp), hence reducing the risk for disruption of the energy supply caused by centralization.
    • Total available energy: ~1700 TW (~20% of this is realistically harvestable, given the low insolation at high latitudes and competing land uses)
    • Capacity factor: 0.1-0.2 (depending on location, cloudiness, tilt, efficiency)
    • Lifetime: ~30 years
    • Energy payback time: 1-3 years
    • Planning-to-operation time: 2-5 years
    • Total emissions: 19-59 g CO2e/kWh (depends on insolation and amount of energy used in production)
Wind energy
  • Essence of wind: Convert the kinetic energy of moving air (wind) into rotational energy in a turbine to generate electricity.
    • Wind physics
      • The instantaneous power density of wind (usually 100-1000 W/m2) is proportional to the cube of the instantaneous wind speed. Finding locations with high wind speeds is thus crucial for successful wind power generation.
      • Wind speeds at a given height follow a Weibull distribution, which for wind can be approximated quite accurately by the Rayleigh distribution (a special case of the Weibull with shape parameter k=2).
      • Due to shear forces (the friction of air flowing across rough land), wind speeds increase roughly logarithmically with height: Taller wind turbines are thus more efficient. Modern turbines typically have a tower height of ~80 m (the hub or axis of rotation is at that height).
    • In 2007, 94 GW of peak wind generation capacity was installed globally; actual energy production satisfied ~1% of the ~2 TW of global electricity demand.
    • Wind energy requires the smallest footprint of all renewable technologies, although the total land area required is larger due to the array spacing between turbines needed for efficient operation (i.e., each turbine decreases the wind speed behind it, reducing the power density available to nearby turbines).
      • The area required for each turbine is approximately A = 4D x 7D, where D is the rotor diameter.
    • Total available energy: ~72 TW (~6 TW in US)
      • Average wind speeds of over 7 m/s are required for wind turbines to be economically competitive with other energy technologies. Such speeds exist over ~13% of global land area.
    • Capacity factor: 0.33-0.35
    • Lifetime: 20-30 years
    • Energy payback time: 2-4 months
    • Planning-to-operation time: 2-5 years
    • Total emissions: 3-7 g CO2e/kWh (lowest of all technologies surveyed)
Hydroelectric energy
  • Essence of hydro: Release the gravitational potential energy of water in an elevated reservoir, allowing it to flow through a turbine.
    • Hydroelectric plants are built on rivers, either by damming (to flood large areas of land and create a reservoir with water storage) or by introducing turbines that do not significantly alter the course of water flow.
    • Pumped hydro (pumping water uphill, storing it, and releasing it through a hydroelectric plant when electricity is needed) is the only energy storage technology that has been successfully demonstrated at the utility scale (~100 GW, compared to hundreds of MW for all other storage technologies), with storage efficiencies of 70-90%.
    • Hydropower is the largest installed renewable energy source globally, accounting for 17.4% (~36.5 GW) of total electricity production in 2005. Hydroelectric plants are often used as peaking plants since they can start producing power quickly (15-30s when in spinning-reserve mode) to match demand variations and smooth the intermittency of other generation sources—hydropower thus complements other renewables like solar and wind.
    • Total available energy: ~2 TW (5% has been tapped)
    • Capacity factor: 0.42
    • Lifetime: 50-100 years
    • Energy payback time: ~1 year
    • Planning-to-operation time: 8-16 years
    • Total emissions: 16-61 g CO2e/kWh
Ocean energy: Wave and tidal
  • Essence of wave energy: Convert the kinetic energy of wind-driven waves on the surface of the ocean to mechanical motion in a floating generator.
    • The power of a wave is proportional to the density of water (more dense => larger mass (per unit volume) => more kinetic energy), the group velocity of the wave (which is proportional to the wave period), and the height of the wave squared (the intensity or power of any wave is proportional to the amplitude squared).
    • Total available energy: 480 GW
      • Given that 2% of the world’s ~8×105km of coastline has wave power density greater than 30 kW/m
    • Capacity factor: 0.21-0.25
    • Energy payback time: 1 year
    • Planning-to-operation time: 2-5 years
    • Total emissions: 40-60 g CO2e/kWh
  • Essence of tidal energy: Use undersea turbines to harness the energy of oscillatory undersea currents caused by the gravitational attraction of the moon and the sun.
    • The ocean is continuously transitioning between high tide and low tide, with 4 such transitions (2 in each direction) every day. This predictability (6 hours in each direction) makes tidal energy a potential baseload generation source.
    • Tidal currents must have a speed greater than ~2 m/s for tidal power to be economical—lower than the 7 m/s required for wind energy, since water is more dense than air.
    • Tidal turbines are usually mounted on the sea floor, with the rotors either directly exposed or preceded with a narrowing duct to direct water toward them.
    • Total available energy: ~800 GW (~20 GW practical)
    • Capacity factor: 02.-0.35
    • Energy payback time: 3-5 months
    • Planning-to-operation time: 2-5 years
    • Total emissions: 34-55 g CO2e/kWh
Geothermal energy
  • Essence of geothermal: Drive water deep into the ground, allowing the thermal energy in the Earth’s crust to heat it up, and use the hot water to drive a steam turbine.
    • Geothermal energy arises from radioactive decay and planetary formation.
    • Leading geothermal technologies: Dry steam, flash steam, binary
      • Dry and flash steam systems are used when geothermal reservoir temperatures are 180-370ºC or higher; both require the drilling of two boreholes, one for steam (dry) and/or liquid (flash) flowing upward and one for condensed water flowing downward.
        • The dry steam approach uses (vapor-phase) steam pressure to drive a turbine directly.
        • The flash steam approach (most common today) converts hot liquid water into steam in a low-pressure flash tank, which then drives a turbine.
        • Neither system (currently) condenses greenhouse gases (CO2, NO, SO2, H2S) released from the underground reservoir during extraction, instead releasing them to the atmosphere and exacerbating climate change.
      • Binary cycle systems (~15% of current systems) are used when reservoir temperatures are 120-180°C.
        • Hot water rising from a borehole is used to heat and evaporate a low-boiling point “binary” or “working” organic fluid (e.g., isobutene, isopentane); the resulting vapor is used to drive a turbine.
        • Since they are closed-loop systems, binary cycle plants do not emit any greenhouse gases into the environment.
        • Because they can be operated at intermediate temperatures, binary systems may be the most promising and flexible of the geothermal approaches.
    • Total available energy: ~170 TW (but only ~100 GW at reasonable cost)
    • Capacity factor: 0.73 (good)
    • Lifetime: ~30 years
    • Energy payback time: 10+ years
    • Planning-to-operation time: 3-6 years
    • Total emissions: 16-60 g CO2e/kWh

Nuclear energy

  • Essence of nuclear (fission): Use slow neutrons to split the nuclei of heavy elements (e.g., U-235, Pu-239) into high-energy products (e.g., Kr-92, Ba-141, neutrons, gamma rays), which collide with and boil water to drive a steam turbine.
    • The most common nuclear fuels are uranium-235 and plutonium-239.
      • Uranium is typically stored as small ceramic pellets in metal fuel rods, which are used in a reactor for 3-6 years before being replaced.
      • The radioactive waste products resulting from nuclear fission must be stored in proper containment to isolate it from the biosphere for many thousands to millions of years (i.e., until the radioactive elements have decayed sufficiently).
    • The US has more nuclear power plants than any other country (~25% of the world’s total), followed by France, Japan, and Russia.
    • Total available energy: 0.4-14 TW (lower number for once-through thermal reactors, higher number for light-water and fast-neutron reactors)
    • Capacity factor: 0.81 (really good)
    • Lifetime: ~40 years
    • Energy payback time: 1-2.5 years
    • Planning-to-operation time: 10-19 years
    • Total emissions: 68-180 g CO2e/kWh
Fossil energy with carbon capture and storage (CCS)
  • Essence of CCS: Burn fossil fuels (coal in particular, since it’s abundant) but capture and store the resulting carbon emissions to reduce the impact on climate and air quality.
    • Emitted CO2 can be captured and injected deep underground in geological formations (e.g., saline aquifers, depleted fossil fuel fields, unminable coal seams) or into the deep ocean.
      • Worldwide, geological formations have the potential to store up to ~2000 Gt CO2, while we emit ~30 Gt CO2 every year. Clearly CCS is not a permanent solution.
      • Deep ocean sequestration of CO2 makes the ocean more acidic due to the formation of carbonic acid (H2CO3), and the sequestered CO2 eventually equilibrates with surface levels and is re-emitted into the atmosphere.
        • This option has been dismissed by most credible sources.
      • An alternative CCS approach is to combine CO2 with common metal oxides (e.g., quicklime (CaO), MgO, Na2O) to form solid carbonate minerals (e.g., CaCO3, MgCO3, Na2CO3) that can be easily sequestered. This approach requires a lot of raw oxide material and large energy inputs to speed up the reaction (via high temperatures and pressures).
    • Carbon capture technologies can reduce CO2 emissions by up to 85-90%.
      • But the energy required to capture and store CO2 will significantly reduce the power output (by 10-40%) of a power plant equipped with CCS technology.
      • And non-CO2 pollutants are still emitted, at even higher rates than before (more fuel must be burned to generate the same power output).
    • No major power plant today has successfully implemented carbon capture and storage.
    • Total available energy: ~1 TW for 200 years (~60% of annual electricity production)
      • Based on proven coal reserves
    • Capacity factor: 0.65-0.85
    • Lifetime: 30 years
    • Energy payback time: ??
    • Planning-to-operation time: 6-11 years
    • Total emissions: 310-570 gCO2e/kWh
Bio energy: Corn and cellulosic ethanol (E85)
  • Essence of biofuels: Let plants (or animals) convert solar energy into chemical energy, then burn the plants (or animal dung) or decompose them into liquid ethanol for transportation fuels.
    • Biofuels can be solids, liquids, or gases derived from organic matter: wood, grass, straw, manure, corn, sugarcane, wheat, sugarbeet, or molasses. These materials can be burned directly (e.g., for heating, cooking, and transportation) or converted into liquid ethanol for transportation.
      • The sugars and starches in crops are used fermented by microorganisms to create ethanol. Cellulose from switchgrass, wood, or miscanthus (a fast-growing grass) can also be converted into ethanol through fermentation, although the process is far slower than for the crops mentioned previously.
      • E85 fuel consists of 85% ethanol and 15% gasoline.
    • The amount of land required for bioethanol production is massive: We would need to triple the US land area dedicated to corn farming (to ~10% of the 50 states) just to power all US vehicles with corn ethanol.
      • The demands on freshwater for irrigation would be similarly immense.

The conclusion?

Wind and solar energy, coupled with baseload hydro and a bit of geothermal, can turn the climate change equation upside down, slashing emissions while improving the long-term sustainability of our energy supply. Biofuels and CCS do more harm than good. Existing technologies can make a big difference already; technology advances will make renewable technologies more and more attractive by both economic and climate-change metrics.

So where do we go from here?

Start reading. Start learning. Start protesting.

Thanks for reading!

And thanks to my ONE Labmates—in particular Patrick and Geoffrey—for many frightful discussions on energy and climate change.

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