In our previous post we briefed you on the energy storage potential in the United Kingdom. With Brexit, Italy will become the third largest member state after Germany and France. With extensive mountain terrain in the north, Italy has long been dependent upon hydroelectric generation. Until the mid 1960s hydropower represented nearly all electricity production in Italy. The installed capacity of hydropower has been stagnant since the mid 1960s, with a rapid growth in fossil fuel based generation driving the overall share of hydropower fall from ~90% to 22% in 2014. A detailed breakdown of electricity sources in Italy is shown below.
Electricity Production in Italy (2014)
Considerable effort has been made to transition Italy to a low carbon electricity sector. As of 2016, Italy had the 5th highest installed solar capacity in the world and the 2nd highest per capita solar capacity, behind only Germany. In addition to its impressive solar progress Italy ranks 6th worldwide in geothermal with 0.9 GW.
Italy’s solar growth was propelled by feed-in-tariffs that wer enacted in 2005. This provided residential PV owners with financial compensation for energy sold to the grid. However, the feed-in-tariff program ceased on 06 July 2014 after the €6.7 billion subsidy limit was reached.
Even with its impressive accomplishments in renewable energy, traditional thermal generation (natural gas) still account for ~60% of total electricity generation in Italy. How much effort will go into reducing this number is still unclear. Italy has committed to 18% renewables by 2020 and is nearly 70% of the way there already so there is little urgency on reducing fossil-based electricity from the perspective of meeting this target. However, Italy is heavily reliant on fossil fuel imports (Deloitte) and energy security requirements will likely continue to push the development of more domestic electricity sources like renewables.
Energy Storage Facilities
Italy is dominating the electro-chemical energy storage market in Europe. With over 6,000 GWh of planned and installed electro-chemical generating capacity (~84 MW installed capacity), Italy is far ahead of 2nd place UK. This is largely due to the massive SNAC project by TERNA (Italy’s TSO), a sodium-ion battery installation totaling nearly 35 MW over three phases. A breakdown of energy storage projects, by technology type can be seen below.
Italy is one of the top markets in the EU for energy storage and is primed for growth. The Italian TSO, TERNA, has been investigating selling energy storage as a service. In 2014 the AEEG, the electrical regulator under which TERNA operates, proposed that batteries should be treated as generation sources similar to cogeneration plants. Italy has always been a market completely dominated by a small number of big centralized utility companies and this trend is likely to continue when it comes to EES deployment. These companies have been focusing their efforts on battery technologies and are expected to continue down this path.
However, the private market could present great opportunity for P2G. The International Battery & Energy Storage Alliance have summarized the reality of Italy’s untapped energy storage market as follows: “With high solar output of 1,400 kWh/kWp, net residential electricity prices around 23 cent/kWh and currently no FIT, the Italian energy market is considered to be highly receptive for energy storage.”
Italy is now well-stocked with residential PV systems that can no longer collect subsidies. Combine this with the fact that the vast majority of homes in Italy burn natural gas imported from Russia, Libya and Algeria and it is clear that Italy presents a unique opportunity for P2G at a residential/community level. This is echoed by Energy Storage Update who in 2015 concluded that Italy was “one of the top four markets worldwide for PV-and-battery-based energy self-consumption.”
While it is unclear exactly how many residential PV systems there are in Italy, it was speculated in late 2015 that there were over 500,000 PV plants in Italy.
How do we shape human exploration on Mars to minimize what we must bring with us and to maximize the value and utility of what we bring, and augment it with what is already there?
To provide sufficient fuel for a safe return of the Mars crew, we can produce methane and oxygen on Mars which could be aided by microbes that are brought to Mars. We suggest lightweight perovskite solar panels that may be transported to Mars. In the optimistic scenario, about 18 months are required for the installation of surface solar power and fuel production for the failsafe return after which the crewed mission commences. The pessimistic scenario takes 4 years. To save oxygen, we also suggest Martian perchlorates as rocket fuel component. For later missions’ food supply, we suggest to use lichens as primary colonizers to produce organic rich soil.
Procedure to produce energy on Mars
We suggest the use of in situ Martian resources for the production of methane as ascent and return propellant. Since oxygen generated during electrolysis is not sufficient for a safe return, we also suggest to use algae for the production of oxygen. Algal biomass will be used as soil for food production. Methane producing microbes will be contained in methanogenic electrolysis reactors (MER) while algae will grow in covered craters. The production of methane on Mars is to be carried out autonomously by robots and reactors that will land near the ice-rich polar regions to melt water as electrolyte for low temperature electrolysis. The Mars lander will autonomously construct facilities with the purpose of propellant production to fuel the Mars transfer vehicle which enables transport between Mars surface and Earth orbit. Only when enough propellant for a safe return to Earth has been produced, shall a manned Mars mission begin. Furthermore, methane will be used as energy storage should there be failure of energy collectors such as solar panels. Goal is to erect a 3.5 MW solar power plant on Mars by the end of the first manned mission.
Schematic of the fuel production process. Red circles highlight the end products steel (top) and CH4/O2 (bottom). Top: 1, Lander on ice, 2, Nuclear fission reactor (NFR) with heat exchanger to melt ice, 3, mining unit, 4, iron/nickel ores, 5, induction steel foundry with energy supply from the NFR (2), 6, algae enrichment tank with water supply from the NFR (2), 7, crater algae reservoir producing O2 and biomass for 8, dehydrator separating water and 9, biomass pellets, 10, carbon steel for Bottom: 11, solar heat collector melting ice and producing water for 12, microbial electrolysis reactor (MER) producing methane and oxygen collected by 13, degasser, 14, solar panels mounted on carbon steel producing electricity for MER (11), 15, gas storage tank, 16, Mars orbit transfer vehicle fueled by CH4/O2
To sustain the life of 6 crew members a power production capacity of 170 kW (see “Surface habitat energy needs”) is required and has highest priority along with fuel production for a safe journey home. The procedure is sketched out in the figure above. It is a stepwise process during which most steps are interdependent and therefore self-accelerating as power production increases. To minimize risk of failure, we recommend at least four independent landers in the circum-polar region of Mars. The polar regions bear the highest surface water content which is crucial for manned explorer missions, methanogenic electrolysis as well as the production of oxygen and biomass. The landers will carry a small nuclear fission device that will begin mining for iron and titanium ores to produce the steel required as structural support for solar panels. First, the lander begins mining for iron ores so that steel production can begin. Graphite or other reduced forms of carbon for steel production will be shipped from Earth because organic matter production on Mars by algae is a slow process. Alternatively, lightweight modular carbon fiber elements to mount solar panels will be brought from Earth to Mars. Once the 170 kW solar plant is established, melting ice for the methanogenic electrolysis reactors (MER) begins. Combined power from the nuclear fission reactor (NFR) and the solar plant will be used. Only when the amount of methane for a failsafe return (Orion capsule size) has been produced, power will be diverted into three equal parts: continue to melt ice for algae, start or continue to mine ores for thermal collectors, continue to produce methane. After enough thermal collectors are produced to pace ice melting with algal growth, electrical ice melting will be shut off. Now, electrical energy is used for steel production to install more solar panels and methane production until sufficient for a comfortable (more payload) return. By this time, the crew is arriving and decides what the highest priorities are. We recommend to focus on accelerating algal growth for oxygen and biomass production as complete independence from Earth requires the production of organic carbon from CO2.
The composition of Martian soil as analyzed by NASA’s Curiosity and other rovers (Source: NASA 2012)
Energy need for the Mars Transfer Vehicle
Fuel produced on Mars will serve 3 purposes:
Failsafe: return of one Orion-sized vehicle
Desired: production of return fuel allowing for less stress (more comfort) during transit
Energy storage at night or power failure
Two options for Mars-Earth return seem possible. (1) Option one was suggested by NASA’s DRA 5.0 and involves one Orion-like vehicle of about 12 tons and a travel speed of 14 km/s. This option requires only one vehicle but provides less comfort for the long journey home and hence, it exposes the crew to higher stress. However, it uses less fuel therefore enables a faster completion of the first Mars mission. (2) We envision a second option that involves two vehicles, one Orion-like lift-off vehicle for transport of 6 crew members into a Mars orbit of 250 km and one transit vehicle to return to Earth. As option two is the preferred option, we recommend to use option one, the Orion-only scenario, as failsafe scenario.
We assume the capacity of a manned Orion capsule with Dragon thrusters (Draco) as reference. The Orion-sized vehicle can carry 6 crew members and has a weight of 12 tons including fuel. For a Mars lift-off, a thrust-mass ratio of at least 5 N/kg will be needed resulting in 60 kN thrust or 150 Draco thrusters to lift 6 crew members. The travel time from Martian surface to a 250 km orbit would be 7 minutes at full thrust. About 600 m3 methane (at Earth atmospheric pressure) would be required for the lift-off. To produce this amount of methane at 210 kW (40 kW nuclear fission and 170 kW solar power, see “Surface habitat energy needs”), 3 years of dedicated fuel production are necessary. The proposed solar power installations of 1,400 m2 perovskite solar cells can produce effectively 170 kW during a Mars day of 8h (that is, 20 kW/m2 can be captured by perovskite panels). Using the same Orion capsule also for the Mars-Earth transit, another 7 minutes or 17,000 km are required to reach the travel speed of 14 km/s and approximately the same for full stop. To produce enough fuel for the Mars-Earth transit, only 3 more days are required. Once enough fuel for the failsafe scenario has been produced, the crew will leave the lower Earth orbit heading for Mars.
For the comfort scenario, we assume the proposed 63-ton crewed payload option from the Mars DRA 5.0 used for Earth-Mars transit. This seems reasonable as most equipment will be left behind on Mars and only the transit habitat, the Orion capsule (for emergencies), engines and fuel are needed. This scenario, however, requires considerably more fuel for which the 210 kW surface power generators must produce methane for at least 42 years. Since this is out of scope, we recommend an extension of the surface power production to 3,500 kW which would reduce the required production of methane to 30 months at 14 km/s travel speed or 12 months at 9 km/s. The reduced stress on the crew justifies the lower travel speed and the higher investment. However, 6.3 tons (corresponding to 0.18 km2) of perovskite solar panels will be required to produce sufficient fuel. About 280 tons of steel are necessary as structural support for this scenario. Since 900 kWh/ton of steel will be consumed for electric smelting, only about one year of steel production using a 40 kW NFR (or 2 months using the full combined 210 kW) justify the increased comfort. The shipment of carbon fiber elements form Earth may completely eliminate the need for steel production in this stage of the mission. The process will be self-accelerating as power production increases during the solar panel assembly process.
To produce methane sufficient for one lift-off, a 200,000 liter MER is required using steel mesh/brush electrodes (anode and cathode) of 2,200 m2 projected surface (see figure below). A modular redundancy system of smaller dimension will improve safety but requires more material. MERs have the theoretical capacity to produce sufficient ascent fuel for one Orion capsule within less than one or two days, if power supply were sufficient. At maximum performance, this reactor would consume about 100 GWh or 220 kWh/mol methane. Microbes will facilitate electrolysis at low temperatures and these microbes will be carried by the Mars lander in small (100 mL) redundant batches. Since the limit for methane production is not the reactor capacity but the electrical power available, doubling the amount of solar panels will half the time required for methane production and so on. To store the so produced methane we recommend to first pressurize water to 200 bar before it is injected into the MER. To extract as from the electrolyte, a small pressure reduction is needed and the so obtained gas phase is then conducted into pressurized steel tanks for later use.
An experimental MER needs to be constructed on Earth to prove this concept. Like the Mars reactor, this experimental MER will be a 5 x 5 m cylindrical reactor of either one or two chambers each. The advantage of the two-chamber system is the separation of oxygen and methane but it requires more water while the single chamber reactor is easier to build and holds less water but O2/CH4 separation is required after production. Unfortunately, the ratio of oxygen to methane is difficult to predict as it depends on the anodic pH. A mass ration greater than 2:1 is required. We therefore propose the use of algae as additional oxygen source (see “Photosynthesis crater to produce oxygen and biomass”). As electrodes brush or spiral steel mesh electrodes will be used. Steel mesh (40 x 40 mesh) produced on Mars will be used having a 1,100 m2 projected surface of each electrode.
A detailed description of the reactor can be found here.
Alternative oxidants in cold methane fuel cells or rocket fuel
It is anticipated that oxygen scarcity imposes severe limitations on any manned Mars mission. Oxygen is crucial as propellant and for any human presence. The use of methane for energy storage makes only sense if there is an adequate electron acceptor. While methane can be burnt in turbines at acceptable efficiencies for electricity production, it may also be used in fuel cells. However, no catalysts exist that oxidize methane on electrodes at room temperature or below. The only possible exception are anaerobic methane oxidizing consortia that naturally use biological electron transport chains. The use of biological electron transport chains opens the possibility to capture the energy stored in electrons during transport. Since this is electron acceptor independent, oxidized metal minerals, which are abundant on Mars, can be used as electron acceptors. The disadvantage of this methane fuel cells is that less energy will be captured compared with oxygen. Also, they only exist in theory.
The high acidity on Mars, however, is in favor of reaction, forming additional water from iron oxides and protons using the reductive power of methane. Soluble Fe2+ may be used for electric steel production as the reduction of Fe2+ to Fe0 requires a considerably lower redox potential and therefor lower energy.
Martian perchlorate salts may serve as oxidant in rocket fuel. Ammonium perchlorate and, on Mars, the more prevalent calcium perchlorate are explosive oxidizers. To transform calcium perchlorate into the ammonium salt, ammonium can be produced by a variety of microbial process such as nitrogen fixation (by way of the nitrogenase enzymes) and catabolic ammonification of amino acids or waste urea (by the urease enzyme). Should collection and compression of the photosynthetically derived O2 gas prove impractical for in rocket fuel, our solid oxidizer approach is also submitted. This dual oxidizer strategy will provide for far greater flexibility and more breathable oxygen. Mined perchlorate may also be used to disinfect water.
Photosynthesis crater to produce oxygen and biomass
Oxygenic biophotolysis of water using psychrophilic (cold-loving), dinitrogen fixing cyanobacteria, i.e. blue green algae, grown in covered craters is one proposed plausible means of generating the need for oxygen and biomass. The surplus of oxygen will be required as propellant and component of artificial air in the SHAB. While initially not crucial for a Mars mission, the production of organic matter is useful for more extended missions with larger teams and longer presence. Organic matter is essential for rich soil which, in turn, is pivotal for vegetable food production on Mars. Moreover, cyanobacteria and algae require little engineering and energy which makes them ideal for autonomous production of utility compounds such as organic matter and oxygen.
The amount of damaging cosmic rays and UV can be higher due to the lack of an ozone layer and protective magnetosphere. The amount of cosmic radiation (est. 0.076 Grays per year) is certainly within the tolerable range for many Earthly microbes as it is only around what the interior of the international space station is exposed to. UV light, with its shorter wavelength, can be readily blocked by a thin covering of Martian soil whereas longer wavelengths of photosynthetically active radiation can penetrate further. The microbes will be selectively enriched in their survival zones. Alternatively, a UV protective cover could be used over the crater. The lightweight but durable and robust crater coverings could take the form of an inflatable inverted dome anchored around the crater edge by cables and spikes. The clear upper canopy would admit sunlight but have coating to block harmful radiation while the curved lower surface could be reflective (to maximize photosynthesis) or black to absorb heat. Solar powered gas pumps could adjustably increase the internal gas pressure to accelerate carbon and nitrogen fixation rates and water accumulation from the trace water vapor available.
Conversion of limited amount of solar energy and frozen water plus copious CO2 into biologically generated oxygen plus organic matter will require a phototroph capable of survival at extremely low temperatures consistent with the Martian surface. We propose to identify terrestrial cyanobacteria capable of this by selectively enriching them from mixed biofilm consortia obtained from the Earth’s Arctic and Antarctic regions. Samples obtained from rocky coastal brines will be subjected to intensive evaluation in selective enrichment freezers outfitted to replicate the polar Martian habitat. The finding that the lichen Pleopsidium chlorophanum (gold cobblestone lichen) can survive, adapt and grow under Martian environmental conditions bodes well for this approach.
During the initial surface resource utilization phase (see figure above), the growth of algae is the most time consuming step and therefore the production of liquid water has highest priority. The use of craters will eliminate the need for containers for growing and there reduce the amount of material brought to Mars. Ideally, such craters are equatorial flat water ponds that ensure maximum sunlight capture and minimum water reheating. These preconditions do not align with the initial mission setup (landing near polar ice caps) but should be prepared during the first mission. That is, water pipes from the circum polar regions to the equatorial areas must be constructed. The pipes may need to be heated which requires additional energy or heating the melted water to high temperatures and pressures to prevent ice formation during transport.
Production of water as medium for methanogenic electrolysis and algae
The lack of liquid water is a major hindrance since active metabolism requires a fluid aqueous medium. In addition to the production of methane fuel, melting ice is the greatest challenge for the first manned Mars mission. Liquid water is essential for MERs and algae craters. Hence, all excess of heat or electrical power produced should be directed to melting of ice after methane fuel production is secured. The obtained CO2-rich brine is the electrolyte for MERs. The high acidity is not inhibitory for microbial growth as acidophilicmethanogens and algae were reported from terrestrial environments. Fortunately, the low pH will reduce the electrical overpotential needed for hydrogen generation, which is the intermediate step during methanogenic electrolysis. The low pH, on the other hand, inhibits oxygen formation which is why corrosion of steel anodes is anticipated to become a possible problem. Anode corrosion must be monitored and shall not exceed a certain, yet to be determined, threshold before spent anodes are recycled in steel foundries brought with the first mission.
The low temperatures on Mars that reach only 20°C in equatorial regions also impose a major hurdle on liquid water maintainance. That is, water may need to be heated by parabolic heat collectors to remain liquid. However, Fischer et al. recently found that “when the salts are in contact with water ice, liquid brine forms in minutes, indicating that aqueous solutions could form temporarily where salts and ice coexist on the Martian surface and in the shallow subsurface.” If our crater canopy is fitted with an internally reflective coating in the infrared spectrum small green houses can be created an the brine will remain liquid longer.
The MERs use methanogenic microorganisms for methane production which will be brought to Mars along with algae seeds by the landers. The methanogenic microbes are highly efficient in methane production, resulting in electricity capturing efficiencies close to 100%. Precious metal catalysts are not required. In contrast, for effective oxygen production, platinum or palladium coating may be required on the anodic side of the MERs. Anodic algae appear to be an alternative but need to be further explored. Since the amount of platinum used is very low, it may be transported as salt to Mars and electroplated on steel electrodes once they are ready. Electroplating is an easy procedure so that a robot can accomplish this task within few minutes. However, platinum recycling requires 1-2 days of work of one crew member.
About 280 tons of steel for structural support of solar panels are required (see “Production of steel for structural support of Mars surface components”). The carbon content of steel should not exceed 2.1% to guarantee high stability and therefore we chose 1.5% carbon for Mars steel. That is, about 4 metric tons of carbon are required for steel production. This is the bottleneck of steel production. Assuming cold conditions on Mars comparable to the Antarctic, a good approximation for biomass concentration in brine is 5 mg/m3. At this concentration, nearly 1 billion m3 water need to be processed. While the existing amount of 821,000 km3 would be more than sufficient, it is impossible to melt this amount of ice within the mission’s timeframe using an NFR of 40 kW even if other power sources were counted in. Therefore, parabolic heat collectors could be shipped from Earth as well. At an energetic efficiency of parabolic heat collectors of 80%, 300 tons would be required to melt this amount water within 2 years. Using 10 tons of parabolic collectors, one can melt only 26,000 m3 for algae during 2 years. This is enough to produce 130 g algal carbon in little more than 2 years assuming a constant concentration of 5 mg/m3. It is more efficient to bring 4 tons of graphite to Mars for initial steel production or consider reflecting surfaces other than polished steel.
Alternative use of covered craters to accumulate water using native perchlorates
In light of the considerable difficulties associated with installing long water pipelines to fill craters with water, we outline an elegant alternative strategy for gradually capturing water from the atmosphere using native perchlorates in the Martian sediments.
Perchlorate salts have been detected in Martian sediments and craters such as the Dale Crate and at concentrations of 0.5-1% globally. Calcium perchlorate is an extremely hygroscopic component of the Martian soil that was recently discovered to cyclically draw H2O from the Martian atmosphere into the soil by night to form saline liquid brine. By sealing the crater covers by day, when the water normally sublimates off, and then opening in-current or one way valves by night after photosynthetically formed O2 has been recovered, atmospheric water vapor can slowly be accumulated as brine liquid/ice inside of the crater at zero or minimal energetic cost.
Halophilic algae tolerate high salt concentrations and low temperatures. Due to the high concentration of CO2 in the native atmosphere the crater covering will amplify the warming effects of this greenhouse gas to prolong the duration of liquid state water needed for nitrogen and carbon fixation. By transporting more soil-derived perchlorates, possibly with crushed water ice deposits, into the covered crater, water can slowly be accumulated. Biologically formed nitrous oxide gas might further accentuate the internal greenhouse warming and thus biological rates of activity.
Perchlorates salts from inside the crater can be recovered from the liquid brine to gradually reduce the salinity of the water. This could be done using parabolic evaporation troughs periodically lifted above the briny surface. Since perchlorates are considered a human toxin, they can be removed by some microbes such as perchlorate reducing bacteria (PRBs) which use percholorates as alternative electron acceptors. Such PRBs could be introduced at a later stage to eventually render the water-filled covered craters non toxic to higher forms of life.
Soil conditioning through phototrophic primary productivity
Lichen and blue green algae have both been used as foods on Earth for hundreds of years. Spirulina is one example of a widely consumed cyanobacterium that uses sunlight to synthesize essential vitamins, antioxidants like beta-carotene and fatty acids from CO2. One major advantage of using a nitrogen fixing cyanobacterium is that they can use solar energy to convert atmospheric nitrogen gas directly into the essential amino acids that future manned missions will need to build and maintain muscle on the Red planet. This will reduce the amount of fuel spent on shuttling food supplies in. Surprisingly, some species of cyanobacteria contain 60% protein per dry gram which is more protein than beefsteak, without the high amount of deleterious cholesterol. Gaseous nitrogen makes up roughly 2.7% of Mars’ thin atmosphere and is available globally. Nitrogen gas is not the only bioavailable form of nitrogen needed to grow oxygenic phototrophs. Nitrates are an ideal fertilizer. The Curiosity rover identified bioavailable nitrates as a significant component of the sediment on Mars. Trace elements are also present in rocks and soil but may require processing.
Production of steel for structural support of Mars surface components
Steel cannot be brought to Mars as there will be at least 2.2 tons of steel necessary for structural support for 1,400 m2 perovskite solar panels. While lightweight carbon fiber modules could be used as structural support, it is possible to produce steel in situ. Steel production on Mars seems an apparent alternative to transport of construction material given the abundance of iron, nickel and titanium on Mars. However, it also requires organic carbon which is to be produced by CO2-fixing algae that grow first in enrichment tanks (transparent plastic bags) and later in covered craters. After dehydrating the algae medium, recycling and reheating it, dry algae pellets will be used as supplement for steel production. The dehydration and reheating process require additional energy which can be provided as heat using parabolic collectors. Parabolic collectors are more efficient in terms of energy capturing and easier to construct as polished steel can be used as opposed to organic Pb/I composites in perovskite solar cells. Steel is then shaped and polished to build parabolic thermal collectors to melt more ice and provide more energy until the NFR and solar panels can be fully replaced by parabolic collectors that can also produce electricity. The production of steel is limited by the amount of organic carbon available. Therefore, we recommend to explore the possibility to use methane gas as reductant and carbon source for steel production. Methane gas production is faster and requires less water resources than algae.
The Mars landers will also mine iron ores and silicates for the production of wires, solar panels and construction materials. Steel will be produced in an induction furnace using iron ores and graphite or organic biomass. Organic biomass from algae tanks will be used for steel production. This organic biomass will also be used for graphite production at a later stage of the mission. Alternative furnace concepts are possible. For example, methane can be used as reductant. Another alternative would be an electric arc furnace or sacrificial graphite electrodes. Graphite can be produced from organic carbon as follows
Organic carbon from CO2 by cold adapted algae
Organic carbon + 800ºC → C
C + SiO2 + 1,400ºC → SiC
SiC + 4,200ºC → graphite
The steel and graphite induction furnaces will be carried by the landers
Energy for initial steel production for the construction of the 170 kW solar plant (see “SHAB energy needs”) is produced by an NFR. A 40 kW reference NFR is recommended. Steel production from iron ore using electric smelting requires 900 kWh per ton of steel. That is, to produce enough structural support for solar panels for 6 people, about 2,000 kWh are necessary or about 3 days of energy production at full performance. This is based on the assumption that steel of 2 mm thickness and 10% of the perovskite area of 1,400 m2 is sufficient. To produce enough structural support for 3.5 MW (0.18 km2) perovskite solar cells needed for the comfortable return option, 280 tons of steel are required. At 50 days of steel production using the entire 210 kW (40 kW NFR + 170 kW solar perovskite life support) are anticipated. To add 1.5% carbon, 4 tons of graphite are necessary which will be carried as cargo from Earth.
Steel is necessary for parabolic heat collectors on site as well. Parabolic heat collectors are required for melting ice for algae growth. The production of steel sufficient for parabolic collectors to melt 1 billion m3 of ice is approximately 600 tons, i.e. 9 additional tons of graphite need to be shipped. To make this amount of steel on Mars another 2 years will be necessary at least. This appears to be the best trade-off between cargo transport and waiting time for a Mars mission but is still an approximation. The algal side product oxygen, also justifies this approach. Since the launch of a manned mission is not dependent on algae production, it is not counted as wait time, which would add another 4 years prior to launch. This shall only demonstrate the feasibility of in situ steel production on Mars. As alternative, the use of methane produced on Mars as carbon and electron source for steel needs to be explored as this may eliminate the need for graphite transport or carbon fiber construction materials are transported to Mars.
Possible perovskite production and reuse of lead produced by the nuclear fission reactor
Lead remnants from the 235U nuclear fission reactor aboard the landers can be used as they are a side product of the radioactive decay of contaminating 238U. There are no confirmed higher concentrations of iodine on Mars and this element needs to be brought in the form of elemental iodine, KI or NaI with the lander to produce the PbI and methyl ammonium iodide. However, since iodine can be replaced by the element chlorine for perovskite production, the initial amount iodine may not need to be replenished because chlorine is an abundant element on Mars.
Solvents required for perovskite cells can be produced in situ using methane gas and acetic acid (also a possible side product of MERs) as precursors as soon as they become available. To bridge the intermediate gap, solvents and organic reactants brought to Mars by the landers may be used. These materials are:
As synthesis of perovskite cells may still be to difficult on Mars, we recommend the transport of these components. As shown below (Surface habitat [SHAB] energy needs), only 9 kg perovskite solar panels will be required. As photo voltaic systems are constantly improved, one can expect better performance and lighter weights in future.
Devices included in this chart of the current state of the art have efficiencies that are confirmed by independent, recognized test labs (NREL, AIST, JRC-ESTI, and Fraunhofer-ISE) and are reported on a standardized basis (Source: NREL 2019)
Surface habitat (SHAB) energy needs
The average energy need per capita in the European Union was 150 GJ/year. Although this estimate is likely too high for a permanent colony on Mars we used this as reference herein. A perovskite solar panel operating at 12% efficiency can produce 14 MJ/day/m2 from Mars aphelion 8 hour solar radiation assuming 170 W/m2. That is, 240 m2 methyl ammonium lead halide perovskite solar panels are required to sustain the presence of one person on Mars. This requires 350 g mesoporous TiO2, 370 g Au per capita. The light weight of about 720 g per capita makes a transport of these components from Earth to Mars feasible (8.5 kg total). Mounting the thin solar power collector on a stable steel surface can be achieved on Mars but only if steel is produced in situ. NASA’s Human Exploration of Mars Design Reference Architecture recommends a mission of 6 explorers. That is, at about 1,400 m2 perovskite solar panels or 170 kW capacity will be required only to sustain life of one manned explorer mission. Before installation of these panels, steel will have to be produced on which they can be mounted. For the envisioned larger energy needs, 3.5 MW are necessary and for this, about 6.3 tons of perovskite may be shipped to Mars.
In our last post about the EU energy storage market we gave a brief overview of Germany’s situation. Now, we show how the United Kingdom prepared itself for its energy transition. Traditionally, the UK’s energy mix has been dominated by fossil fuels. This remains the status quo today, as approximately 60% of the electricity generated in the UK comes from fossil fuel sources, with another 20% coming from nuclear.
UK electricity production 2015 (Source: The UK Government)
While the UK has been heavily dependent on carbon-intensive sources of electricity, in 2008 they committed to a 15% renewable energy target (by 2020) and 80% reduction in CO2 emissions (by 2050; Department of Energy & Climate Change). However, the UK has stated that they will miss the 15% renewable target for 2020, due to the lack of properly designed policy measures. There has been considerable pressure to transition to a low carbon market and with one-quarter of existing generating capacity (mainly coal and nuclear) expected to close by 2021; it is expected that growth in renewable energy will lead to more energy storage capacities.
The UK has made excellent progress on its short-term clean energy goals and there is optimism that this trend will continue. Large-scale development of low carbon generation technologies such as wind and solar is expected to continue.
Energy Storage Facilities
As of late 2016, there were 27 non-PHS EES plants representing 430 MW of installed capacity in the UK (Sandia National Laboratories). The UK’s energy storage portfolio is dominated by electro-chemical based technologies (primarily lead-acid and lithium-ion battery installations). This is shown below.
Number of Existing & Planned Energy Storage Facilities in the UK, by Type (Source: Sandia National Laboratories)
As was shown for Germany, only a very small fraction of EES facilities are dedicated to renewables capacity firming. The existing EES capacity is almost exclusively dedicated to critical transmission support (on-site power). While nearly all of the EES capacity under development is dedicated to bulk energy storage (electric energy time shift).
There is still considerable uncertainty around the growth of EES in the UK, and with such a small sample size it is difficult to infer any correlation from the data in the figure above. According to the previous UK government, however, being geographically isolated and a net importer of electricity, one would expect the UK to place a heavier focus on renewables capacity firming in the long-term.
Energy Storage Market Outlook
The UK is in the midst of a major restructuring of their electricity generating portfolio and the market under which these assets operate. With a large portion of the existing capacity due for retirement in the next 10-15 years, the UK faces challenges in meeting energy needs while balancing decarbonization efforts. As part of this, major investment is needed in all areas of the electrical grid, including energy storage.
In its Smart Power publication, the National Infrastructure Commission outlined that while the UK is being faced with challenges to cover aging infrastructure this represents an opportunity to build efficient and flexible energy infrastructure. The Commission stated that energy storage was one of the three key innovations for a “smart power revolution”.
Many other official government bodies have expressed similar thoughts regarding energy storage. In its Low carbon network infrastructure report, the Energy and Climate Change Committee stated that “storage technologies should be deployed at scale as soon as possible”, while urging the Government to eliminate the outdated and unfair regulations that have been handcuffing energy storage development in the UK (Garton and Grimwood).
In April 2016, the Government acknowledged concerns regarding the regulatory hurdles facing energy storage projects (primarily double-charging of network charges) and stated that they would begin working with the National Infrastructure Commission and ECCC to investigate the issue. While there may be regulatory hurdles hindering energy storage in the UK, the Government has shown commitment through funding. Since 2012, the government has contributed over £80 million to energy storage research. In addition to this, the Department of Energy and Climate Change have developed a new £20 million fund to help drive innovation in energy storage technologies.
Overall, the outlook for energy storage in the UK is positive. There is considerable pressure to begin developing energy storage facilities at scale from not only industry, but also many government bodies. Investors are ready as well. As stated by the National Infrastructure Commission: “businesses are already queuing up to invest”.
Simply put: regulatory hurdles are holding back growth in the UK energy storage market. With the Government making major strides in renewable energy development and being vocal about its commitment to making the UK a leader in energy storage technology, these regulatory hurdles will likely be relaxed and there should be considerable growth in the UK energy storage market in the near-term.
At this point, specific technology types and service uses have not been hypothesized in detail. However, with the UK being geographically isolated and a net importer of electricity, logic would suggest an emphasis on renewables capacity firming in the long-term to maximize domestic consumption of renewable energy. Rapidly decreasing costs in electro-chemical technologies, coupled with the fact that much of the existing gas-fired capacity will be reaching end of life by 2030 suggest that the UK EES market would not be ideal for P2G technologies.
Yeasts such as Saccharomyces cerevisiae are, as the name suggests, used for large scale production of beer and other alcoholic beverages. Their high salt and ethanol tolerance not only makes them useful for the production of beverages, but also suitable for the production of combustion fuels at high alcohol concentrations. Besides ethanol, long-chain fusel alcohols are of high interest for biofuel production as well. Bioethanol is already mixed with gasoline and thus improves the CO2 balance of internal combustion engines. This liquid biofuel is made from either starch or lignocellulose. The production and use of bioethanol supports local economies, reduces CO2 emissions and promotes self-sufficiency. The latter is especially important for resource-depleted landlocked countries.
In order to efficiently produce ethanol and other alcohols from lignocellulose hydrolysates, yeasts must use both glucose and pentoses such as xylose and arabinose. This is because biomass is rich in both lignocellulose and thus glucose and xylose. However, this is also the main disadvantage of using Saccharomyces cerevisiae because it does not ferment xylose. Consequently, the identification of another yeast strains capable of fermenting both these sugars could solve the problem. Highly efficient yeasts can be grown in co-cultures with other yeasts capable of lignocellulose fermentation for ethanol production. Such a yeast is, for example, Wickerhamomyces anomalous.
To further improve ethanol production, bioelectric fermentation technology supporting traditional fermentation can be used. The microbial metabolism can thus be controlled electrochemically. There are many benefits of this technology. The fermentation process becomes more selective due to the application of an electrochemical potential. This, in turn, increases the efficiency of sugar utilization. In addition, the use of additives to control the redox equilibrium and the pH is minimized. Ultimately cell growth can be stimulated, further increasing alcohol production.
Such bioelectric reactors are galvanic cells. The electrodes used in such a bioelectric reactor may act as electron acceptors (anodes) or source (cathodes). Such electrochemical changes affect the metabolism and cell regulation as well as the interactions between the yeasts used. Now, a research group from Nepal (a resource-depleted landlocked country) has used new yeast strains of Saccharomyces cerevisiae and Wickerhamomyces anomalous in a bioelectric fermenter to improve ethanol production from biomass. The results were published in the journal Frontiers in Energy Research.
For their study, the researchers chose Saccharomyces cerevisiae and Wickerhamomyces anomalus as both are good ethanol producers. The latter is to be able to convert xylose to ethanol. After the researchers applied a voltage to the bioelectrical system, ethanol production doubled. Both yeasts formed a biofilm on the electrodes, making the system ideal for use as a flow-through system because the microorganisms are not washed out.
Saccharomyces cerevisiae cells in a brightfield microscopic image of 600-fold magnification (Foto: Amanda Luraschi)
The researchers speculated that the increased ethanol production was due to the better conversion of pyruvate to ethanol − the yeast’s central metabolic mechanism. The researchers attributed this to accelerated redox reactions at the anode and cathode. The applied external voltage polarized the ions present in the cytosol, thus facilitating the electron transfer from the cathode. This and the accelerated glucose oxidation probably led to increased ethanol production.
Normally, pyruvate is converted into ethanol in fermentation yeast. External voltage input can control the kinetics of glucose metabolism in Saccharomyces cerevisiae under both aerobic and anaerobic conditions. Intracellular and transplasmembrane electron transfer systems play an important role in electron transport across the cell membrane. The electron transfer system consists of cytochromes and various redox enzymes, which confer redox activity to the membrane at certain sites.
The authors also found that an increased salt concentration improved conductivity and therefore ethanol production. The increased ethanol production from lignocellulosic biomass may have been also be due to the presence of various natural compounds that promoted yeast growth. When the cellulose acetate membrane was replaced by a Nafion™ membrane, ethanol production also increased. This was perhaps due to improved transport of xylose through the Nafion™ membrane as well as the decrease of the internal resistance. A further increase of ethanol production was observed when the bioelectrical reactor was operated with fine platinum particles coated on the platinum anode and neutral red deposited on the graphite cathode.
Several yeast cultures from left to right: Saccharomyces cerevisiae, Candida utilis, Aureobasidium pullulans, Trichosporum cutaneum, Saccharomycopsis capsularis, Saccharomycopsis lipolytica, Hanseniaspora guilliermondii, Hansenula capsulata, Saccharomyces carlsbergensis, Saccharomyces rouxii, Rhodotorula rubra, Phaffia rhodozyba, Cryptococcus laurentii, Metschnikowia pulcherrima, Rhodotorula pallida
In our last posts we introduced electrical energy storage (EES) and the EU market for EES. Now, we focus on some important EU members, beginning with Germany. The country’s electrical energy portfolio reflects its status among the most progressive countries in the world in terms of climate action. As of November 2016, Germany had produced ~35% of its 2016 electricity needs from renewable sources as outlined in the Figure below.
Electricity Production in Germany (Source: Fraunhofer ISE)
However, these numbers are somewhat skewed based on the fact that the electro-mechanical category is essentially two large capacity CAES plants. In reality, electro-chemical projects (mainly batteries) are much more prevalent and represent the vast majority of growth in the German storage market. There are currently 11 electro-chemical type energy storage projects under development in Germany and no electro-mechanical projects under development (see figure below).
As outlined earlier, there are a multitude of service uses for EES technologies. Currently the existing EES fleet in Germany serves grid operations and stability applications (black start, electric supply capacity), and on-site power for critical transmission infrastructure. A breakdown of service uses in the German market is shown below.
Net Exports of Electricity with Average Day-Ahead Market Pricing for Germany in 2015 (Source: Fraunhofer ISE)
Energy Storage Market Outlook
Logic seems to indicate that with aggressive renewable energy targets, a nuclear phase-out, and increased emphasis on energy independence Germany will need to develop more EES capacity. However, many have conjectured that the lagging expansion of EES in the short and medium term will not pose a barrier to the Energiewende. In fact, some claim that EES will not be a necessity in the next 10-20 years. For example, even when Germany reaches its 2020 wind and solar targets (46 GW and 52 GW, respectively), these would generally not exceed 55 GW of supply and nearly all of this power will be consumed domestically in real-time. Thus, no significant support from EES would be required.
The German Institute for Economy Research echos these sentiments and argue that the grid flexibility needed with significant renewable energy capacity could be provided by more cost-effective options like flexible base-load power plants and better demand side management. Additionally, innovations in power-to-heat technologies which would use surplus wind and solar electricity to feed district heating systems present significant opportunity, while creating a new market of energy service companies.
While there is work being done, economically feasible production of P2G is currently not achievable due to limited excess electricity and low guaranteed capacity. This limited excess electricity, is an example of the effect of power exports discussed earlier. While there may not be a significant commercial market in the short-term, introduction of P2G for transport could act as an additional driver behind continued renewable energy development in Germany.
Over the past decade, numerous studies have shown that global warming is roughly proportional to the concentration of CO2 in our atmosphere. In this way one can estimate our remaining carbon budget. This is the total amount of man-made carbon dioxide that can still be released into the atmosphere before reaching a set global temperature limit. The nations of the world agreed on this limit in the 2015 Paris Agreement. It should not exceed 1.5°C, and in any case be well below 2.0°C. However, diverging estimates have been made for the remaining carbon budget, which has a negative impact on policy-making. Now, an international research group of renown climate experts has published a framework for the calculation of the global CO2 budget in Nature. The researchers suggest that the application of this framework should help to overcome the differences when estimating the carbon budget, which will help to reduce uncertainties in research and policy.
Since the fifth report of the Intergovernmental Panel on Climate Change (IPCC), the concept of a carbon budget has become more important as an instrument for guiding climate policy. Over the past decade, a series of studies has clarified why the increase in the global average temperature is roughly proportional to the total amount of CO2 emissions caused by human activity since the Industrial Revolution. In the framework, the research group cites numerous published documents that provide evidence for the linearity of this correlation. This literature has allowed scientists to define the linear relationship between warming and CO2 emissions as a transient climate response to cumulative CO2 emissions (TCRE). The linearity is an appealing concept because of the complexity of the Earth’s response to our CO2 emissions. Additional processes that affect future warming have been included in recent models, among them, for example, the thawing of the Arctic permafrost. These additional processes increase the uncertainty of current climate models. In addition, global warming is not just caused by CO2 emissions. Other greenhouse gases, such as methane, fluorinated gases or nitrous oxide, as well as aerosols and their precursors affect global temperatures. This further complicates the relationship between future CO2.
In the case of global warming caused by CO2, every tonne contributes to warming, whether that ton is emitted in future, now or in the last century. This means that global CO2 emissions must be reduced to zero, and then remain zero. This also means that the more we emit in the next years, the faster we have to reduce our emissions later. At zero emissions, warming would stabilize, but not disappear. It may also reverse. An overdraft of the carbon budget would have to be compensated by removing the CO2 later. One way of removing CO2 from the atmosphere would be a technology called direct air capture, which we reported earlier. Ultimately, this will probably be the only way left, as carbon neutral renewable energy source sources only make up 5% of our energy mix. Establishing a global carbon budget will further highlights the urgency of our clean energy transition. Unfortunately, there is a large divergence when it comes the amount of the CO2 remaining in our carbon budget. In their framework, the researchers cite numerous studies on carbon budgets to maintain our 1.5°C target. Starting 2018, these range from 0 tonnes of CO2 to 1,000 gigatons. For the 2.0°C target, our carbon budget ranges from around 700 gigatons to nearly 2,000 gigatons of remaining CO2 emissions. The aim of the researchers is to limit this uncertainty by establishing a budget framework. The central element is the equation for calculating the remaining carbon budget:
The budget of the remaining CO2 emissions (Blim) for the specific temperature limit (Tlim) is a function of five terms that represent aspects of the geophysical and human-environment systems: the historical man-made warming (Thist), the non-CO2 contribution to the future temperature increase (TnonCO2), the zero emission commitment (TZEC), the TCRE, and an adaptation for sources from possible unrepresented Earth system feedback (EEsfb).
Term
Key choices or uncertainties
Type
Level of understanding
Temperature limit
Tlim
Choice of temperature metrics that allow global warming, the choice of pre-industrial reference and consistency with global climate targets
Choice
Medium to high
Historical man-made warming
Thist
Incomplete data and methods for estimating the man-made component; see also Tlim
Choice and uncertainty
Medium to high
Non-CO2 contribution to future global warming
TnonCO2
The level of non-CO2 contributions coinciding with global net zero CO2 emissions; depends on policy choices, but also on the uncertainty of their implementation
Choice and uncertainty
Medium
Non-CO2 contribution to future global warming
TnonCO2
Climate reaction to non-CO2 forcers, such as aerosols and methane
Uncertainty
Low to medium
Zero-emissions commitment
TZEC
The extent of the decadal zero emission commitment and near-zero annual carbon emissions
Uncertainty
Low
Transient climate response to cumulative emissions of CO2
TCRE
TCRE uncertainty, linearity and cumulative CO2 emissions that affect temperature metrics of the TCRE estimate
Uncertainty
Low to medium
Transient climate response to cumulative emissions of CO2
TCRE
Uncertainty of the TCRE linearity, value and distribution beyond peak heating which is affected by cumulative CO2 emissions reduction
Uncertainty
Low
Unrepresented Earth system feedback mechanisms
EEsfb
Impact of permafrost thawing and duration as well as methane release from wetlands on geomodels and feedback
Uncertainty
Very low
In the CO2 budget, the unrepresented Earth system feedback (EEsfb) is arguably the greatest uncertainty. These feedback processes are typically associated with the thawing of permafrost and the associated long-term release of CO2 and CH4. However, other sources of feedback have been identified as well. This include, for example, the variations of CO2 uptake by the vegetation and the associated nitrogen availability. Further feedback processes involve changes in surface albedo, cloud cover, or fire conditions.
It remains a challenge to adequately characterize the uncertainties surrounding the estimates of our carbon budget. In some cases, the reason of these uncertainties is inaccurate knowledge of the underlying processes or inaccurate measurements. In other cases the terminology is used inconsistently. For better comparability and flexibility, the researchers propose to routinely measure global surface air temperature values. This method gives robust data for models and model runs over selected time periods. More detailed comparisons between published estimates of the carbon budget are currently difficult because the original data used for publication often are missing. The researchers therefore propose to provide these in the future along with publications.
Breaking down the carbon budget into its individual factors makes it possible to identify a number of promising pathways for future research. One area of research that might advance this field is to look more closely at the TCRE. Future research is expected to narrow down the range of TCRE uncertainties. Another promising area of research is the study of the correlation between individual factors and their associated uncertainties, for example, between uncertainties in Thist and TnonCO2. This could be achieved by developing methods that allow a more reliable estimate of historical human-induced warming. It is also clear that less complex climate models are useful to further reduce the uncertainties of climate models, and hence the carbon budget. Currently, each factor of the framework presented by yhr researchers has its own uncertainties, and there is no method to formally combine them.
At Frontis Energy, too, we think that progress in these areas would improve our understanding of the estimates of our carbon budget. A systematic understanding of the carbon budget and is crucial for effectively addressing global warming challenges.
In our previous post of this blog series on Electrical Energy Storage in the EU we briefly introduced you to different technologies and their use cases. Here, we give you a short overview over the EU energy grid. Supplying approximately 2,500 TWh annually to 450 million customers across 24 countries, the synchronous interconnected system of Continental Europe (“the Grid”) is the largest interconnected power network in the world. The Grid is made up of transmission system operators (TSOs) from 24 countries stretching from Greece to the Iberic Peninsula in the south, Denmark and Poland in the north, and up to the black sea in the east. The European Network of Transmission System Operators (ENTSO-E) serves as the central agency tasked with promoting cooperation between the TSOs from the member countries in the Grid. The ENTSO-E, in essence, acts as the central TSO for Europe. With over 140 GW of installed wind and solar PV capacity, the EU trails behind only China in installed capacity. A breakdown of the individual contributions of EU member states is shown below in the figure above.
Energy Storage in the EU
For this study a number of European countries were selected for more detailed investigation into energy storage needs. These countries were selected based on a combination of existing market size, intentions for growth in non-dispatchable renewable energy and/or energy storage, and markets with a track record of innovation in the energy sector.
On a total capacity basis (installed and planned MW) the top three energy storage markets within the EU are: Italy, the UK, and Germany. These countries were selected on the basis of these existing market sizes.
Spain and Denmark were selected based on their large amounts of existing renewable energy capacity and − in the case of Denmark − the forecasted growth in renewable energy and energy storage capacity.
Each of the selected countries (Germany, UK, Italy, Spain, Denmark, Netherlands) are discussed in the proceeding sections, providing a more detailed overview outlining their current electricity portfolios and decarbonization efforts, current energy storage statistics, and a brief discussion on market outlook.
Researchers from the National Renewable Energy Laboratory (NREL) have developed a cheap method for producing high-octane gasoline from methanol. They recently published their method in the journal Nature Catalysis. Methanol can be synthesized from CO2 via various routes, as we reported last year. Biomass, such as wood, is one possibility.
The production of biofuels from wood, however, is too expensive to compete with fossil fuels. To find a solution to this problem, the researchers combined their basic research with an economic analysis. The researchers initially aimed at the most expensive part of the process. Thereafter, the researchers found methods to reduce these costs with methanol as an intermediate.
So far, the cost of converting methanol to gasoline or diesel was about $1 per gallon. The researchers have now reached a price of about $0.70 per gallon.
The catalytic conversion of methanol into gasoline is an important research area in the field of CO2 recovery. The traditional method is based on multi-stage processes and high temperatures. It is expensive, producing low quality fuel in small quantities. Thus, it is not competitive with petroleum-based fuels.
Hydrogen deficiency was the initially problem the researcher had to overcome. Hydrogen is the key energy containing element in hydrocarbons. The researchers hypothesized that using the transition metal copper would solve this problem, which it did. They estimated that the copper-infused catalyst resulted in 38% more yield at lower cost.
By facilitating the reintegration of C4 byproducts during the homologation of dimethyl ether, the copper zeolite catalyst enabled this 38% increase in product yield and a 35% reduction in conversion cost compared to conventional zeolite catalysts. Alternatively, C4 by-products were passed to a synthetic kerosene meeting five specifications for a typical jet fuel. Then, the fuel synthesis costs increased slightly. Even though the cost savings are minimal, the resulting product has a higher value.
Apart from the costs, the new process offers users further competitive advantages. For example, companies can compete with ethanol producers for credits for renewable fuels (if the carbon used comes from biogas or household waste). The process is also compatible with existing methanol plants that use natural gas or solid waste to produce syngas.
Electrical Energy Storage (EES) is the process of converting electrical energy from a power network into a form that can be stored for converting back to electricity when needed. EES enables electricity to be produced during times of either low demand, low generation cost, or during periods of peak renewable energy generation. This allows producers and transmission system operators (TSOs) the ability to leverage and balance the variance in supply/demand and generation costs by using stored electricity at times of high demand, high generation cost, and/or low generation capacity.
EES has many applications including renewables integration, ancillary services, and electrical grid support. This blog series aims to provide the reader with four aspects of EES:
An overview of the function and applications of EES technologies,
State-of-the-art breakdown of key EES markets in the European Union,
A discussion on the future of these EES markets, and
Applications (Service Uses) of EES.
Table: Some common service uses of EES technologies
Storage Category
Storage Technology
Pumped Hydro
Open Loop
Closed Loop
Electro-chemical
Batteries
Flow Batteries
Capacitors
Thermal Storage
Molten Salts
Heat
Ice
Chilled Water
Electro-mechanical
Compressed Air Energy Storage (CAES)
Flywheel
Gravitational Storage
Hydrogen Storage
Fuel Cells
H2 Storage
Power-to-Gas
Unlike any other commodities market, electricity-generating industries typically have little or no storage capabilities. Electricity must be used precisely when it is produced, with grid operators constantly balancing electrical supply and demand. With an ever-increasing market share of intermittent renewable energy sources the balancing act is becoming increasingly complex.
While EES is most often touted for its ability to help minimize supply fluctuations by storing electricity produced during periods of peak renewable energy generation, there are many other applications. EES is vital to the safe, reliable operation of the electricity grid by supporting key ancillary services and electrical grid reliability functions. This is often overlooked for the ability to help facilitate renewable energy integration. EES is applicable in all of the major areas of the electricity grid (generation, transmission & distribution, and end user services). A few of the most prevalent service uses are outlined in the Table above. Further explanation on service use/cases will be provide later in this blog, including comprehensive list of EES applications.
Cost-effective and efficient methods for converting sunlight into electricity are the focus of green energy research. Solar cells developed for this purpose are currently made of semiconductors such as silicon. Electrical energy is generated at the junction between two different semiconductors. However, the efficiency of these solar cells has almost reached its theoretical limit. New methods of converting sunlight into electricity must be found if solar energy is to be used as a major source of electricity. An international research team from Germany, Japan and Israel has now made important progress in this direction. Zhang and colleagues recently published their findings in the prestigious journal Nature. They demonstrate a transition-free solar cell that can be made by applying a more atomic semiconductor layer into a nanotube.
In a conventional solar cell, two regions of a semiconductor are doped with different chemical elements. The electrical current is generated by the negatively charged electrons of a region and by the positively charged electron holes (holes). At the junction between these two areas, an electric field is created. When sunlight is absorbed at this junction, electron-hole pairs are formed. The electrons and holes are then separated by the resulting electric field, generating an electric current. This conversion of solar energy into electricity is called photovoltaic effect. This photovoltaic effect is particularly important for green energy production. Its efficiency has almost reached the theoretical limit as mentioned above.
The reported bulk photovoltaic effect (BPVE) is based on tungsten disulfide, a member of the TMD family. Crystals of this material have a layered structure and can be stratified in layers similar to graphite. The resulting atomic sheets can then be rolled into tubes of 100 nanometers by chemical methods. The authors produced photovoltaic devices from three types of tungsten disulfide: a monolayer, a bilayer and a nanotube.
A systematic reduction in crystal symmetry has been achieved beyond mere fractional symmetry inversion. The transition from a two-dimensional monolayer to a nanotube with polar properties has been significantly improved. The photovoltaic current density produced is orders of magnitude greater than that of other comparable materials. The results not only confirm the potential of TMD-based nanomaterials, but also the importance of reducing crystal symmetry for improving the BPVE.
While the nanotube devices had a large BPVE, the single-layer and two-layer devices produced only a negligible electric current under illumination. The researchers attribute the different performance characteristics of the solar cells to their pronounced crystal symmetry. This way, one can spontaneously generate a current in uniform semiconductors, without a transition.
The BPVE was first observed in 1956 at Bell Labs, New Jersey, just two years after the invention of modern silicon solar cells. The effect is limited to non-centrosymmetric materials characterized by a lack of symmetry in spatial inversion. That is, the combination of a 180° rotation and a reflection. The effect has two attractive properties: the current generated by light depends on the polarization of the incident light and the associated voltage is greater than the band gap of the material. This is the energy required to excite conducting free electrons. However, the effect typically has a low conversion efficiency and was therefore of rather academic than industrial interest.
To achieve high efficiency, a photovoltaic material must have high light absorption and low internal symmetry. However, these two properties usually do not exist simultaneously in a given material. Semiconductors that absorb most of the incident sunlight generally have high symmetry. This reduces or even prevents the effect. Low-symmetry materials, such as perovskite oxides, absorb little sunlight due to their large band gap. To circumvent this problem, efforts have been made to improve light absorption in low-symmetry materials, for example by using the mentioned doping. Meanwhile, it has been shown that the effect can occur in semiconductors by using mechanical fields to adjust the crystal symmetry of the material.
The newly discovered solution is encouraging with regard to the production of high absorption semiconducting nanotubes. In the case of tungsten disulfide, the crystal symmetry of the nanotubes is reduced compared to the mono- and bilayers due to the curved walls of the tube. The combination of excellent light absorption and low crystal symmetry means that the nanotubes have a significant photovoltaic effect. The current density exceeds that of materials which are inherently low in symmetry. Nevertheless, the conversion efficiency achieved is still much lower than that of the photovoltaic effect in conventional junction-based solar cells.
The authors’ findings demonstrate the great potential of nanotubes in solar energy production and raise various technological and scientific challenges. From an application’s perspective, it would be useful to produce a solar cells that consists of a large arrays of semiconductor nanotubes to check whether the approach is scalable. The direction of the generated current would be largely determined by the internal symmetry of the material. Therefore, uniform symmetry across the nanotube array would be required to create a collective current. These currents could cancel each other out.
At Frontis Energy, we wonder if the method described could work with the classic photovoltaic effect in the same solar cell. That would possibly increase overall efficiency. The two effects could use the solar energy consecutively. Despite the remaining challenges, the presented work offers a possibility for the development of highly efficient solar cells.
