Physics of Sustainable Energy 3 – Live Coverage
Solar Upconversion by Jen Dionne, Stanford University & FOM-AMOLF
- Low-energy photon utilization for upconversion to increase PV efficiency
- “One photon, two photon: red photon, blue photon”
The solar spectrum is spread out among the following regimes: 5% UV, 43% visible and 52% infrared. Upconversion combines low-energy photons, ideally in the infrared, to create new higher energy photons that can then be absorbed by the cell above the cell.
Upconversion models are modeled as three-level energy system. Energy states created in the bandgap, real and imaginary, and are then combined under radiative re-combination. The upconverter acts as an LED for the photons so that the light can be manipulated.
The upconverter can increase the ideal cell efficiency from 33% in single junction cells to about 44% with an ideal cell bandgap blue-shift from 1.1ev to 1.7-1.8eV. This could lead to significant material changes in future PV modules. Solar concentration is not needed for upconverters to work, meaning that they work under any light.
Ideal upconverter position between 800 nm and 1200 nm. Promising materials are bimolecular systems, PdOEP and DPA. The PdOEP molecule creates energy states inside the materials allowing for energy transfer through chemical bonds. The DPA then emits that energy from PdOEP to radiative decay, meaning it will emit light. The process is about 20% efficient. The absorption spectrum of the above molecule system is in the visible light range. The aim is to design bimolecular systems that can absorb and convert energy in the infrared range.
Upconversion is currently in the scientific research stage due to low efficiency of the energy conversion and Prof. Dionne has some ideas on how to improve the system efficiency to make it technologically viable:
- The energy transitions in the chemical system are difficult to complete due to quantum-mechanical reasons. One way to potentially circumvent this problem is to change the symmetry of the chemical structure. This can be done by synthesizing nanoparticles that can be mechanically manipulated.
- A second approach is using nanophotonics, meaning using nanoparticles to manipulate light and direct to the structures the system needs it to go. The particle has a core-shell structure, with the upconverting material being at the core and the nanophotonic material being the shell.
Biofuels – Status and Prospects, Chris Somerville (EBI, UC Berkeley)
4 ways of doing solar:
- Solar Thermal
The conversion efficiency from sunlight to biomass is about 1%. Biomass combustion, ie biofuels, can be carbon neutral, but it depends on CO2 reduction. As a result, biofuels are often carbon positive, but have a better carbon efficiency than fossil fuels.
Brazil and the United States ethanol production, sugar cane in Brazil and corn in the US, have significantly increased in the last 50 years. The global biofuel production is ~42 million gallons per day, with Brazil and the US carrying over 90% of the capacity. The current biofuel energy infrastructure is about 40x the size of solar. Energy crops have different energy returns, with corn being really low, ~1.5, prompting researchers to develop new crops to create biofuels.
Miscanthus gigantaeus: An energy crop: The crop goes in Illinois without irrigation or fertilizer. The crop yields about 200 gallons of biofuel per acre. A study in the UK yielded that the energy crop has no response to fertilizer and needs very little mineral nutrients from the soil, indicating that the crop is sustainable for growing.
EBI has 16 farm locations in the United States to grow energy crops and the effects of the crops on the surrounding environment, particularly the agricultural effects. Studies also indicate that the energy crop can grow on land that is not being used for food production, which will avoid land use conflicts. A study concluded that globally there is a 1.5 billion acres available to grow the energy crop.
Desert plants could also expand the availability of land for biomass production. In the semi-ariad Agave, Madagascar, plants grow with low environmental needs, meaning high water and nutrient efficiency. The plant can then be converted into biofuels.
One of the main challenges of biofuel production is to create efficient and low cost conversion processes from the biomass to the actual biofuel. The conversion processes should yield a biofuel price that is competitive with fossil fuels without major subsidies. A major chemical challenge is to convert different types of sugar contained in the plant simultaneously. EBI found a fungal organism that can create the yeast to convert the plant sugars. Chemical reactions used to convert biofuels may also be used to store hydrogen in chemical bonds for later application.
Fun facts: Plants use about 200 grams of water to produce 1 gram of biomass.
Wind Energy – John O. Dabiri, Caltech
Wind energy has a capacity of ~20GW, which is more than 2x of global energy consumption. Even though wind is often considered a mature industry, only very few countries get more than 10% of their energy from wind. The key issues of increasing wind energy capacity is cost. Common cost metrics are the levelized cost of electricity and the capital cost of electricity.
The main goal of the wind turbine is to maximize power production. There are horizontal axis turbines (HAWT) and vertical axis turbines (VAWT). HAWT have a higher power production efficiency than VAWT due to the physics of the energy generation, but they may not be the most effective way to increase wind power penetration. Modern turbines have ~50% efficiency in extracting the kinetic energy contained in the wind, with the Betz efficiency limit being ~60%.
Dabiri took a different approach to efficiency. His starting point was wind power into wind farm x total turbine swept area, trying to capture what fraction of energy flux that goes into the wind farm volume is actually converted to electricity. He then went on to describe simulations to model this new type of efficiency metric. The calculations show that the Betz limit does not account for the wind farm efficiency holistically, as it does not account for interactions between neighboring wind turbines.
Dabiri modeled the flux of wind with computational fluid dynamic methods. The upper limit from the calculations is 68W/m2 assuming wind is flowing at 8m/s. The average modern wind farm generates ~2.5W/m2. This is important as countries with small land areas need high land use efficiency. The following image of an off-shore wind farm illustrates this problem:
Conventional wind energy also has significant logistical challenges in transporting large wind blades through existing infrastructure. Additional challenges include societal acceptance, such as impact on birds, visual and acoustic signature. In the developing there is also an additional challenge of creating an energy infrastructure to support wind energy generation.
Dabiri argues that the VAWT are a better fit for the holistic efficiency of wind farm, since they are
- Scalable for utility power
- Safer for societal concerns
Key challenges for VAWT are lower wind speeds and smaller size, which often to significant decreases in overall efficiency. Dabiri used bio-inspired engineering to address these challenges. He studied the movement of fish schooling, in which fish swim in large swarms. The fish create a similar situation as the wind turbines, as both are displaying significant amount of fluids. In the fish schools, the fish actually use less energy to swim when compared to swimming individually, as they can take advantage of vortexes created by other fish. In the case of wind farms, one important conclusion from that design is that efficiency would be improved if wind turbines rotated in different directions to take advantages of vortexes.
Caltech set up a research wind farm (Caltech Field Laboratory for Optimized Wind Energy) to study how the turbines would perform in the field, where there are a lot more factors one has to account for. The results show that the VAWT would actually outperform the traditional modern HAWT farms, which was a pleasant surprise since the study only aimed to show that they would be comparable. Dabiri current studies focus on modeling the actual flow of the wind through the turbines using computational tools and creative experimental setups, in which he can test smaller scale models of the turbines. A new bio-inspired idea of studying mass fluxes sea grass beds suggests that turbines should be placed with slightly different heights for better performance. Dabiri then presents a cost analysis of the two turbine models. While the HAWT has 800 parts per unit, the VAWT has 12 parts per unit, which should lead to significant cost reductions. He is currently working on setting up wind farms in remote Alaskan villages , which currently pay $0.90 / kwh called “Distributed Power for Alaskan Villages.” Dabiri says that he is also putting the wind turbines under water, and states that the technology is compatible even though the legal environment would be more challenging.
Synergies of Energy and Information Technologies – Eric Brewer, UC Berkeley
Applying Technology for Rural Electrification
In India about 30%-50% of urban electricity is lost or stolen, leading to $4.2 billion loss with the government often looking the other way on the issue. Even though there are large rates of theft in electricity in urban areas, rural areas are a more difficult challenge due to the lack of infrastructure with many uneven loads. In rural areas, the power quality is also poor, leading to voltage spikes that destroy the electricity equipment. UPS regulators cannot be applied in these situations due to economic reasons.
Brewer conducted a study in Kenya, which showed that it could be more affordable to just connect households to the grid instead of building microgrids. The reason why households decide not to connect is because it costs ~$400 per house, which is a significant economic barrier. Brewer decided to collect data on electricity usage of Kenyan households to study how loads are distributed.
In India, the situation is more complicated due to less established policies which aim to drive electricity prices lower for political reasons. Brewer’s student started a company called Gram Power to develop microgrid in India with the first microgrid launching in 2012. Gram Power set up a prepaid model, where the meter is loaded up with credits that are sold by local entrepreneurs. Gram Power also created a low-cost smart meter to develop an accurate billing system that can report loss or theft by measuring current draws. Whenever power theft occurs, the company shuts down the entire village, which has substantially decreased overall power theft. Newer version of the meter also show how much time is left at present current draw, which has allowed the people to be more energy efficient based on the “time left” data. The increase in energy efficiency was a pleasant by-product, in addition to the educational component initiated by the overall power system. Brewer’s study with Gram Power also showed that the microgrid generated cleaner power than the larger grid due to smaller distances the electricity has to travel and the cleaner power source, such as solar photovoltaics.
Brewer emphasizes that electrification has to be redefined to include the quality of power, and that the improved quality of clean power shows the value of solar. Another major conclusion is that small communities people do not steal from their neighbors, which has helped the development of microgrids.
Jonathan Koomey, Stanford University
Energy Use and the Information Economy
The three key points form this talk
- The direct consumption of information technology is modest
- Information technology can help set up a sustainable grid with smart modeling of supply and demand
- Sustainable and efficient energy use more constrained by people and institution then by technology
Electricity use by IT originates mainly from data centers, core network access networks, and end-user communications equipment. Data flows over the internet have changed from voice communications to fixed internet and in 2014 mobile data is making a big dent with project of ~20% of data flow. Data flow has also increased by 1000 times since 1980. The growth in IT is not necessarily a growth in electricity use. What matters most in IT energy usage are user PC’s, data centers and access networks. End-user equipment include computing devices. A smart phone will use ~2-3 kWh per year, while a desktop can use ~200-300 kWh per year. The efficiency of devices is increasing as ultra-low power computing continues to grow. The energy efficiency of computing increases ~100x every decade and doubles about every 1.5 years, leading to tablets and mobile computing. In terms of PC’s, desktops are leveling off in their usage, while laptops continue to grow.
The emissions impact for electronics come mostly from the production phase, except for servers, whose emission originate primarily from the use phase. Data centers consume ~1.3% of global electricity and account for ~2% of US electricity consumption. Most of the electricity consumed by data centers is waste, which creates significant room for efficiency improvement. Recent data also suggests that server installment has slowed down since 2008 due to more advanced computing capabilities. In data centers, older servers can consume ~60% of electricity while only delivering ~4% of the computing capabilities.
Data center GHG emissions are affected by infrastructure efficiency (PUE) and IT efficiency, which is the most important factor. Maximal IT efficiency can save money and power by optimizing current and future energy generation processes. The capital costs of data centers is about $15 million/MW of IT power installed. The largest data center inefficiencies are in enterprise data centers and originate mostly from institutional practices and not from technological developments. One of the main reasons for institutional inefficiencies is the separation of business units from IT units, including separate budgets for the two departments.
Data on the internet show that while GDP grew, growth electricity and energy consumption, as well as GHG emissions, of the internet actually declined. Data also indicates that downloading small data packets, such as music, from the internet is more environmentally friendly than physically transporting them in a CD or other medium. IT advancements can also improve institutional efficiency, such as Walmart who optimized their supply chain using IT. Overall, better data collection and more system effects efficiency case studies to better asses the energy footprint of IT devices and its intertwined effects on energy usage of other sectors. The Second Machine Age is a recommended book for further reading.
The Rebound Effect – Tilman Santarius, UC Berkeley
Energy Efficiency Rebound Effects
In the OECD countries, energy efficiency has increased by 35%, but energy demand has also increased by 20% total. One of the major challenge of energy demand is human behavior, such as buying a more efficient, but also larger, “supersized” fridge. Another challenge is economic growth, as energy demand follows the behavior of GDP. The rebound effect describes an increase in energy demand that has been induced by an improvement in energy efficiency.
Traditionally, the rebound effect has been explained by economics. Two effects are commonly observed in the economics of consumer-side economics, the income effect and the substitution effect. In the income effect, a monetary gain is the motivator to improve efficiency, and the extra income is then used to purchase additional things that lead to the rebound effect. In the substitution effect, a more efficient product has been acquired, but the usage has been increased significantly. Recent studies indicate that the best guess for an average direct rebound effect is between 10-30%. Moreover, 5-10% of income that is not spent on direct rebounds generates indirect rebound effects.
The rebound effect can also be explained using psychological frameworks. Some effects contributing to the rebound effect are:
- Morel Licensing
- Moral Leakage
- Habitual Behavior
- Virtuous Cycle
Most of these effects tend to increase the amount of rebound, but some factors, such as habitual behavior, spill-over and virtuous cycles, can lead to decreases in the rebound effect.
When applying the economic analysis of the rebound effect to the energy space, studies have shown that rebound effects from energy efficiency have contributed to economic growth in OECD economies. The frequency of the rebound effect is still highly debated, and depends strongly on the assumptions made by the writers of each study. This makes it difficult to quantify both the actual occurrence of the rebound effect and how it affects other sectors.
Buildings: Lower Energy, Better Comfort – Gail Brager, UC Berkeley
Buildings are responsible for higher carbon emissions than industry and transportation. In the US, it is estimated that building contribute about 39% of carbon emissions. Yet, the potential for reducing emissions from buildings is really large. Low energy use of buildings should be the first priority, as most energy use occurs during the lifetime of the building. Most energy in buildings is used for space heating and space cooling. Lighting is also a major energy consumer in commercial buildings. Net Zero Energy Buildings (NZEB) has been focusing on reducing carbon emission by measuring performance of buildings in terms of energy and energy cost. In the case of buildings, there are higher priorities for improved energy efficiency than creating renewable energy generation. A good metaphor is is leaky bucket for which the holes have to be fixed first.
Currently, information flow to improve efficiency is a major challenge. Feedback loops between technology, operators, owners, designers, and other personnel is insufficient creating many inefficiencies. A major issue for buildings is to balance energy use and comfort. Particularly in the case of comfort, many decision makers may not care about how efficient their energy use is. Moreover, when looking at 30-year costs of buildings, only 6% make up operations and maintenance, which includes energy. 92% of the cost comes from the personnel, which influences decision makers stronger than energy reductions. Therefore, when marketing building energy efficiency it is critical to link energy reduction effects to effects personnel, which drive the cost of buildings.
Studies about energy consumption vs comfort in buildings show that buildings are being over-conditioned, both in the summer and in the winter. Some data even shows that buildings may be cooler in the summer than in the winter. Furthermore, even though much energy is wasted to over-condition the buildings, many people do not feel comfortable in the conditions, effectively creating a lose-lose situation. One way to address this problem is to use adaptive comfort, in which outdoor situations are re-created. a common example at adaptive comfort is that opening windows increases the range of temperature in which people feel comfortable.
Personal comfort systems aim to switch conditioning to person-based conditioning as opposed to space-based conditioning. In current standards, engineering aims to create thermal monotony, in which an entire space is kept at the same ambient temperature. However, people’s different needs are not accounted for in this system. Similar to having desk lamps, personal conditioning units (PCS) can be used to create comfortable environments for individuals. A PCS can be something as simple as a fan or a foot warmer. Studies show that acceptability of the conditions increased as PCSs were used. Moreover, the energy consumption of PCS is minimal when compared to the savings they provide. New ideas for PCSs include low-energy heated/cooled chairs that are being tested on the UC Berkeley campus.
The idea of alliestesia is to move from thermal monotony to thermal delight, which ties into moving into more personalized conditioning systems. PCS can further increase comfort by modifying temperatures across different body parts. It turns out that having your hands be cooler or warmer in certain situations can actually make you feel more comfortable.
Industrial Ecology – Valerie Thomas, Georgia Tech
Industrial ecology focuses on the interplay of systems and processes in the transition to a sustainable future. The energy sector interacts heavily with many different industries. A key concept in industrial ecology is to study emission “ecosystems” to explore connections across multiple disciplines. Industrial ecology uses various methods to study those systems:
- Material’s Flow Analysis
- Engineering Economics Benefit Analysis to calculate Net Present Value (often used by consulting companies like McKinsey)
- Levelized Cost of Energy (LCOE) Calculations with present energy as opposed to present value
- Life Cycle Assessment (LCA): “cradle to crave” analysis (may also be circular) to address question of energy use, emissions and other related factors
Many LCAs have been qualitative and there are opportunities to improve the studies by adding quantitative metrics for resource and energy use. Moreover, understanding the science behind the process will make the studies more rigorous and the results more impactful.
An LCA an how electric vehicle would affect gasoline use: dx.doi.org/10.1021/es4016926