There has been criticism directed towards the Netherlands for the progress made. According to projections in their 2009 National Renewable Energy Action Plan, the Netherlands should have reached nearly 20% renewable electricity in 2014. This lackluster progress prompted a statement from the EU Commission in its 2017 Second Report on the State of the Energy Union, where the EU Commission stated the Netherlands were the only member state to not exhibit average renewable energy shares which were equal or higher than their corresponding action plan trajectories in 2013/2014.
The EU Commission also stated that the Netherlands was one of the three countries (others: France, Luxembourg) with the biggest efforts required to fill 2020 targets.
Existing Energy Storage Facilities
To date, the Netherlands has almost 20 MW of energy storage capacity either operating (14 MW), contracted (1 MW), or under construction (4 MW).
All energy storage facilities in the Netherlands are electro-chemical, with the exception of the contracted 1 MW Hydrostar underwater compressed air energy storage project in Aruba (Caribbean). Hydrostar is a Canadian company specializing in underwater compressed air energy storage technologies.
The vast majority of the 20 MW of installed energy storage capacity in the Netherlands is spread over just three facilities: the Netherlands Advancion Energy Storage Array (10 MW Li-ion), the Amsterdam ArenA (4 MW Li-ion), and the Bonaire Wind-Diesel Hybrid project (3 MW Ni-Cad battery).
The Netherlands Advancion Energy Storage Array was commissioned in late 2015 and provides 10 MWh of storage to Dutch transmission system operator TenneT. The project, which represents 50% of all Dutch energy storage capacity, provides frequency regulation by using power stored in its batteries to respond to grid imbalances.
The 4 MW Amsterdam ArenA lithium-ion project was commissioned 2017 for PV integration and back up power purposes. The 3 MW Bonaire Wind-Diesel Hybrid project is a battery array located on the Dutch Caribbean island of Bonaire and used as a buffer between intermittent wind energy and the diesel-generation stations on the island.
The remaining 3 MW of Dutch energy storage projects are spread over 21 sub-100 kW facilities, mainly geared towards electric vehicle (EV) charging. Mistergreen, a leading developer of EV charging stations in the Netherlands has constructed 750 kW of LI-ion energy storage arrays at its various electric vehicle charging stations.
Energy Storage Market Outlook
Gearing up for significant market growth for electric vehicles in the Netherlands, there has been a considerable amount of effort to expand the country’s network of quick charging stations. This trend will have to continue in order meet the demand for the 1-million electric vehicles expected in the Netherlands by 2025, so one could expect that there will be large growth in the sub-100 kW Li-ion stations that have already started popping up around the country.
There is little information available regarding the need for large-scale energy storage but the overall need is likely low due to the low penetration of renewables in the electricity sector. However, there is significant focus on energy efficient/independent/self-sufficient housing.
Like Italians, the Dutch are very accustomed to using natural gas in their homes. This, coupled with the push for energy self-sufficient housing could present a unique market for residential power-to-gas systems in the Netherlands.
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.
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.
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.
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.
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.
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.
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.
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.
Electrical energy storage (EES) is not only a vital component in the reliable operation of modern electrical grids, but also a focal point of the global renewable energy transition. It has been often suggested that EES technologies could be the missing piece to eliminating the technical hurdles facing the implementation of intermittent renewable energy sources. In the following blog posts, selected EES markets within the European Union will be evaluated in detail.
With over 80 MW of installed wind and solar capacity, Germany is by far the leading EU nation in the renewable energy transition. However, experts have argued that Germany’s need for widespread industrial scale energy storage is unlikely to materialize in any significant quantity for up to 20-years. This is due to a number of factors. Germany’s geographic location and abundance of connections to neighbouring power grids makes exporting any electricity fluctuations relatively easy. Additionally, when Germany reaches its 2020 targets for wind and solar capacity (46 GW and 52 GW, respectively) the supply at a given time would generally not exceed 55 GW. Nearly all of this would be consumed domestically, with no/little need for storage.
When evaluating energy storage in the UK, a different story emerges. Being an isolated island nation there is considerably more focus on energy independence to go along with their low-carbon energy goals. However, the existing regulatory environment is cumbersome, and poses barriers significant enough to substantially inhibit the transition to a low-carbon energy sector – including EES. The UK government has acknowledged the existence of regulatory barriers and pledged to address them. As part of this effort, a restructuring of their power market to a capacity-based market is already underway. The outlook for EES in the UK is promising, there is considerable pressure from not only industry, but also the public and the government to continue developing EES facilities at industrial scale.
Italy, once heavily hydro-powered, has grown to rely on natural gas, coal, and oil for 50% of it’s electricity (gas representing 34% alone). The introduction of a solar FIT in 2005 lead to significant growth in the solar industry (Italy now ranks 2nd in per capita solar capacity globally) before the program ended in July 2014. In recent years there has been notable growth in electro-chemical EES capacity (~84 MW installed), primarily driven by a single large-scale project by TERNA, Italy’s transmission system operator (TSO). This capacity has made Italy the leader in EES capacity in the EU, however the market is to-date dominated by the large TSOs.
However, the combination of a reliance on imported natural gas, over 500,000 PV systems no longer collecting FIT premiums, and increasing electricity rates presents a unique market opportunity for residential power-to-gas in Italy.
Denmark is aggressively pursing a 100-percent renewable target for all sectors by 2050. While there is still no official roadmap policy on how they will get there, they have essentially narrowed it down to one of two scenario: a biomass-based scenario, or a wind + hydrogen based scenario. Under the hydrogen-based scenario there would be widespread investment to expand wind capacity and couple this capacity with hydrogen power-to-gas systems for bulk energy storage. With the Danish expertise and embodied investment in wind energy, one would expect that the future Danish energy system would be build around this strength, and hence require significant power-to-gas investment.
The renewable energy industry in Spain has completed stagnated due to retroactive policy changes and taxes on consumption of solar generated electricity introduced in 2015. The implementation of the Royal Decree 900/2015 on self-consumption has rendered PV systems unprofitable, and added additional fees and taxes for the use of EES devices. No evidence was found to suggest a market for energy storage will materialize in Spain in the near future.
The final country investigated was the Netherlands, which has been criticized by the EU for its lack of progress on renewable energy targets. With only 10% of Dutch electricity coming from renewable sources, there is currently little demand for large-scale EES. While the Netherlands may be lagging behind on renewable electricity targets, they have been a leader in EV penetration; a trend that will continue and see 1-million EVs on Dutch roads by 2025. In parallel with the EV growth, there has been a large surge in sub-100kW Li-ion installations for storing energy at electric vehicle (EV) charging stations. It is expected that these applications will continue to be the primary focus of EES in the Netherlands.
Similar to Italy, the Dutch rely heavily on natural gas for energy within their homes. This fact, coupled with an ever-increasing focus on energy independent and efficient houses could make the Netherlands a prime market for residential power-to-gas technologies.