Energy storage overview
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Once we’ve covered all the myriad ways of generating electricity, let’s take a look at energy storage. Quite important especially when intermittent energy generation methods like solar and wind become more popular resulting in imbalance between times when energy supply is largest and when demand peaks. The same is true for heat.
Apologies that this is more of a long list of facts than anything else.
Overview
Energy can be stored in numerous different ways.
Electrochemical into batteries. There are three main chemistries used today. Lead-acid batteries store about 30 Wh/kg with 80-90% efficiency. They have a self-discharge rate of 2%/month. Nickel based batteries store 50-80 Wh/kg with 65-80% efficiency. They have the highest self-discharge rate of 10%/month. And lithium based that have highest storage density with 80-150 Wh/kg at 90-100% efficiency. They self-discharge at a rate of 5%/month
Into kinetic energy in flywheels, where a mass rotates around an axis. Faster rotations store more energy. Energy is retrieved by a generator that takes it from the rotating shaft. Typical stored energy is about 5 Wh/kg, high speed systems can achieve 100 Wh/kg. In very high speeds the rotating metal plate may start losing its rigidity, causing design problems.
Into thermal energy. For example, summer’s extra energy can be pumped into the ground to be retrieved by a ground heat pump in winter. Heat can also be stored to building structures (heating buildings to a slightly higher temperature during night when usage is at lowest thus reducing need to heat during day as walls release the stored heat. Or to sand (sand battery), to steam etc.
Into electrostatic energy with super capacitors. The structure of a super capacitor is two separate plates soaked in an electrolyte. Charging causes an electric field between them with one plate being positive and the other negative. There is also very thin dielectric separator to separate the charge between, common choices are paper, carbon, plastic. ‒ Energy density is about 5 Wh/kg. Super capacitors can discharge very quickly with discharge rate between 0.1 – 10 MW
Into potential energy. When intermittent sourced generate excess capacity, water can be pumped to a higher ground reservoir and released when needed. Such reservoirs can be built for example to mountainous areas (pumped storage hydropower plants). Another option is compressed air. Off-peak power could be stored for example in air compressed to an old mine shaft or cavern. When there is demand, pressurised air is released and it drives turbines. Efficiency is around 42 – 55% or with adiabatic method 70%. Generated power level depends on the turbine.
Last but not least energy can be stored in chemical format. There is quite a bit of excitement today around hydrogen. Water can be broken down into oxygen and hydrogen and hydrogen stored for later use. Fuel cells are a good technology for generating electricity from this stored hydrogen. Another option is to additionally capture carbon dioxide from the air and generate methane CH4 (synthetic natural gas). Other options are to store the energy in methanol or DME. Let’s review some of the chemical storage options as we’ve not mentioned them before.
Hydrogen
Hydrogen can be generated from many sources, from fossil fuels, biomass, from water via hydrolysis
In addition to energy store, hydrogen is used as starting molecule in many industrial processes, for example, margarine is made with H2, nickel catalyst and some oils. H2 can also be directly used as fuels, it’s been used in trucks for example. And now there is also clean steel production process using it.
Hydrogen has also a set of its own problems. It is a very small molecule and escapes normal steel pipes. Special steel is needed in containers and pipes. It is liquid in 700 bars making it hard to store. It can be store in pressurised metal tanks or in future in metal hydrides (Metal hydrides are compounds of one or more metal cations (M+) and one or more hydride anions (H−).
Hydrogen needs to be purified for eg. fuel cells. There are a few ways to do this. First one is pressurised swing adaptation (PSA). It is a technique used to separate some gas species from a mixture of gases (typically air) under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperature and significantly differs from the cryogenic distillation commonly used to separate gases. Selective adsorbent materials (e.g., zeolites, (aka molecular sieves), activated carbon, etc.) are used as trapping material, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbed gas. Pressure swing adsorption process (PSA) is based on the phenomenon that under high pressure, gases tend to be trapped onto solid surfaces, i.e., to be "adsorbed". The higher the pressure, the more gas is adsorbed. When the pressure is dropped, the gas is released, or desorbed. PSA can be used to separate gases in a mixture because different gases are adsorbed onto a given solid surface more or less strongly
Or via membrane purification. Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate, or from ceramic materials. Membranes can be used for separating gas mixtures where they act as a permeable barrier through which different compounds move across at different rates or not move at all. The membranes can be nanoporous, polymer, etc. and the gas molecules penetrate according to their size, diffusivity, or solubility. While polymeric membranes are economical and technologically useful, they are bounded by their performance, known as the Robeson limit (permeability must be sacrificed for selectivity and vice versa
Dimethyl ether
Dimethyl ether is another possible energy source. It is clean and easy to handle and can be produced from biomass, methanol, fossil fuels, or methane (natural or synthetic gas). It is a colourless, nearly odourless gas under ambient conditions (room temperature in dry and well ventilated). It is neither a toxic nor carcinogenic compound. It has properties similar to liquid petroleum gas (LPG) and can be easily blended with it and used as a fuel.
In vehicles it is an alternative to diesel. The use of DME in vehicles requires a compression ignition engine (no spark like diesel engines) with a fuel system specifically developed to operate on it.
DME can be obtained in two ways: direct synthesis and through the methanol dehydration process. The production requires only carbon dioxide captured from air (or from flue gases), hydrogen generated from water through electrolysis and power. in methanol dehydration CO2 is first used to produce methanol and then dehydrate it to DME. The logical process is also called Power-to-DME when the speaker does not care about the actual chemical reactions.
DME has some attractive properties. It has a very high cetane number (a measure of fuel’s ignitability in compression ignition engines). Energy efficiency and power rating of DME is very close to diesel. Use of DME eliminates most particulate emissions and possibly removes the need for costly particulate filters. Compared to diesel, it has half the energy density, requiring fuel tanks twice as large.
However, compared to methane, DME has higher energy density and can replace direct methanol fuel cells. This makes DME a promising portable power source. Direct dimethyl ether fuel cell (DDMEFC) is a good choice for a portable power source
Ammonia
Power to ammonia is yet another way to store excess energy. Ammonia itself can serve three different uses. Act as energy store, as feedstock for fertilisers and as fuel for vehicles. Main use today for ammonia is in fertilisers. Nitrogen is one of the essential chemicals that plants require for growth.
As chemical ammonia is toxic and smells bad. But its energy density by volume is double compared to liquid hydrogen and it is easier to ship and distribute.
Ammonia today is made by Haber-Bosch process from natural gas and nitrogen captured from air. It’s the technology that has made fertilizers plentiful and chean and farmers to feed the world’s population. It's estimated that at least half the nitrogen in the human body today comes from industrial ammonia plants. The process happens under pressurized, super-heated steam.
Reverse fuel cell technology is being developed to make ammonia using less energy. As a technology it also lends easier to decentralization. In it waterreacts at the anode to make hydrogen ions (H+), which migrate to the cathode where they react with nitrogen (N2) to form ammonia. The reaction is efficient, but slow.
There is active development to use ammonia as fuel for large ships. Modern ship engines can utilise several fuels – diesel, biofuels, liquidized natural gas (LNG) or soon ammonia. The timing of engine valves is just adjusted to switch. They are also self-learning and can self-calibrate during operations.
Solar Farm is a concept where locally energy is harvest from the environment (via solar etc.). Using that energy ammonia is captured and stored. Part of it is used to make fertilizers for the farm and other part is stored and used during winter or periods when sun don’t shine as energy source is slightly modified combustion engines. It can be used for both electricity and heat generation. And there will be no CO2 emissions as ammonia contains no carbon.
One more approach here.
Fuel cells
Fuel cell are a technology for changing electrochemical energy directly to electricity having a typical efficiency of 50-70%. Fuel cells can be used to produce electricity both in small and big scale. Fuel cell can use many fuels like hydrigen (H2), methanol, hydrocarbons like natural gas etc. Fuel must be purified for use.
As a technology they have many advantages. No moving parts, quiet, relatively high efficiency and long usage periods. However, manufacturing needs platinum catalysator or rare earths for the solid oxide-based variant.
Typical fuel cell voltage is in range 0,6-0,7V. Fuel cells can be stacked in parallel or serial fashion to build the voltage and capacity needed for different applications.
Solid oxide (SOFC) believed to be future, have very high price due to high operating temperature 1000 degrees C. SOFC can use many fuels and does not need noble metals and are very stable in long use. Higher operating temperature enables using more complex hydrocarbons to be used.
Micro fuel cells will provide higher energy density then batteries
A complete system would work so that excess electricity from intermittent sources is turned into hydrogen and then turned back to electricity during high demand. As additional source, biomass can be used to produce hydrogen
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