Storage technologies/research, from physics and chemistry to engineering

Transcription

1 Storage technologies/research, from physics and chemistry to engineering Professor Nigel Brandon OBE FREng Director, Sustainable Gas Institute Director Energy Storage Research Network Co-Director, ENERGY SuperStore Director, H2FC SUPERGEN

2 Predicted duty cycles for grid scale application Two predicted patterns of storage use in a future low carbon grid, showing state of charge against time in hours. The upper curve illustrates the pattern of use for a more distributed storage system, with 6 hours of storage capacity. This equates to around 350 deep cycles per annum The lower curve shows the pattern of use for a more bulk storage system, with 48 hours of storage capacity. This equates to around 250 shallow cycles per annum. Strategic assessment of the role and value of energy storage systems in the UK Low Carbon Energy Future, Report for Carbon Trust; G Strbac et al, (2012) Energy Futures Lab Imperial College London.

3 Energy vs Power The value of storage is not strongly affected by increases in storage duration beyond 6 hours (shown here is a 10 GW case in base case scenario in 2030, Strbac et al for Carbon Trust). Distributed storage initially gains more from an increase in energy at a given power than bulk storage. Low cost solutions are needed in both cases as energy requirements increase. Strategic assessment of the role and value of energy storage systems in the UK Low Carbon Energy Future, Report for Carbon Trust; G Strbac et al, (2012) Energy Futures Lab Imperial College London.

4 Fast storage for frequency regulation Although the market for fast storage (e.g. flywheels, supercaps), is not as large as bulk or distributed storage, the value and savings are substantial and come from a significantly reduced need to run conventional generation part loaded and hence enhanced capability of the system to absorb renewable generation. Strategic assessment of the role and value of energy storage systems in the UK Low Carbon Energy Future, Report for Carbon Trust; G Strbac et al, (2012) Energy Futures Lab Imperial College London.

5 Storage Technology Options Energy storage technologies can be clustered into those that deliver: Mostly energy (pumped hydro, compressed air, flow batteries, hydrogen, liquid air, pumped heat) Mostly power (capacitors, flywheels) Both power and energy (batteries) Of the latter there are high temperature batteries (Sodium-sulphur, sodium nickel chloride) and low temperature batteries (lithium ion. nickel metal hydride, lead acid), with the former often offering better cycle life but requiring energy to keep them at the correct temperature when not in use. We are likely to need storage services for both power and energy, hence a range of safe and affordable storage solutions must be developed, capable of sufficiently long lifetimes at sufficiently low cost.

6 Energy storage technologies deployed on grid Analysis of the UK energy system indicates that installed energy storage capacity in the UK alone could reach 25 GW by 2050.

7 Current collector Current collector 7 Lithium Ion Batteries e le c tron s d is ch a rging c harg ing V D IS CHARG IN G CHARG IN G E L E C T R O LY T E graphite Li Co O charge 6C xli xe Li x C 6 discharge discharge Li 1 x CoO 2 xli xe LiCoO 2 charge

8 Lithium Ion Battery Materials

9 Lithium Ion Battery Packs Lithium batteries can be charged and discharged at high rates but careful voltage and temperature control is required to reduce degradation and ensure safe operation. For safety reasons overcharge must also be avoided, so a relatively sophisticated BMS is required. Also to reduce degradation, extended operation at full charge, full discharge, and high temperatures/currents must be minimised. Element Energy report for CCC (2012) estimated battery pack costs of $792/kWh for a 22 kwh Li-ion battery pack for EV application, based on a cell cost of $400 $/kwh.

10 Tesla Powerwall UK installation of 7kWh unit by Solar Plants, Wales. In the USA installed costs are $6500 per unit. The daily cycle 7 kwh battery ($3000) uses nickel-manganese-cobalt Li-ion and can be cycled 5000 times before warranty expiration. The Tesla Powerwall has a 92.5% round trip efficiency when charged or discharged by a V system at 2 kw with a temperature of 25 degrees, and when the product is new. The 10 kwh battery ($3500) uses a nickel-cobalt-aluminium cathode like the Tesla Model S, is for weekly or emergency use and has a cycle life of around cycles. This product has recently been dropped by Tesla.

11 Lithium Ion Batteries for grid applications Advantages Global supply chain is driving down battery costs. The advantages of Li-ion batteries over other battery technologies are high energy density ( Wh / L), high voltage (>3 V), high round trip efficiency (>93 %), together with low maintenance and relatively low self-discharge. Challenges Technology costs are still relatively high, evidenced by the close to $1000/kWh installed cost for the Tesla Powerwall. Lifetime remains a major challenge, with limited cycle life. Safety is a concern, requiring sophisticated battery management. Better battery management and control needed, especially over long term operation where SOC and SOH become difficult to determine. Lithium availability could be a concern for large scale uptake for grid applications

12 Examples of Lithium (and Sodium) Ion Battery Research Graphite continues to be the dominant anode in Li-ion batteries but suffer degradation. Cells with titanate based anodes are now in the market for grid applications and offer superior safety, high rate and high cycle life compared with graphite, but at the expense of energy density and cost. Understanding SEI formation on graphite anodes is crucial, as the SEI affects the performance of Li-ion batteries in terms of power, capacity and cycle life. The main strategy to improve this is through the use of additives which enhance the properties of the SEI, thus avoiding electrolyte degradation and the loss of lithium capacity. Structured electrode are being developed which aim to deliver both energy and power. Continued materials research is needed to deliver low cost and longer lifetime materials systems for grid applications. To overcome the safety issue in Li-ion batteries, polymer electrolytes are being developed. These are safer than their liquid counterparts, since they are not flammable. Na-ion batteries represent a potentially attractive storage technology at a lower cost for grid scale applications. The price for Na 2 CO 3 (120 $ / ton) is approximately 30 times lower than Li 2 CO 3 (4,000 $ / ton). These are typical raw materials for manufacturing cathodes for Na-ion and Li-ion batteries, respectively. Sodium is much more abundant than lithium in the Earth s crust (23,600 ppm of Na compared with 20 ppm of Li). For grid applications there is less concern that the gravimetric and volumetric energy densities are theoretically lower for a sodium system than those for a directly equivalent lithium system.

13 Flow Batteries Redox Flow Batteries, also termed regenerative fuel cells, are an electrochemical storage technology, but one which decouples power and energy, unlike a usual battery. Electrical energy is stored via the generation of a physically separated reductant and oxidant, and electrical energy generated when required by the recombination of this redox couple. Sumitomo Electric at Yokohama Works. Redox flow battery (1 MW /5 MWh) connected to 28 solar power units of 200 kw total power.

14 Vanadium Flow Batteries

15 Flow Batteries for grid applications Advantages The ability to decouple power and energy, offering flexibility for a wide range of applications requiring either high power or high energy. RFBs have long lifetimes with a proven capability to operate over >10,000 s charge / discharge cycles at deep discharge and >100,000 cycles at shallow discharge. Low levels of self-discharge. Can respond rapidly when charged with electrolyte. High depth of discharge possible. Challenges Technology costs are still relatively high, but do depend on storage capacity. PNNL has published a useful model to assess RFB costs, which estimates near-term costs for a VRFB system at around 4,000 $ / kwh for a 1 MW / 0.25 MWh system and around 475 $ / kwh for a 1 MW / 4 MWh system. Stack power density much lower than equivalent fuel cell technology, adding to cost. Vanadium cost is relatively high. Electrolyte has a relatively low energy density of around 50 Wh / L, meaning that to attain an energy output of 1 MWh, the system requires 20 m 3 of each redox solution, i.e. tanks with around 2 m diameter and 6 m height.

16 Examples of Flow Battery Research

17 Supercapacitors Supercapacitors bridge the power and energy gap between dielectric capacitors and Li-ion batteries. They consist of a porous separator sandwiched between two (usually porous C) electrodes, immersed in a liquid electrolyte (shown in the charged state). They store charge by electrostatic adsorption of electrolyte ions in an electric double layer at the electrode surface.

18 Supercapacitors, capacitors & Li ion batteries ABB has agreed to supply three substations in Dubai, each containing supercapacitor banks and power transformers rated at 50 megavolt amperes (MVA) that will improve the stability and quality of power supply. The substations are planned to be integrated with one of the largest solar parks in the world.

19 Supercapacitors for grid applications Advantages Supercapacitors are best suited to high power and short term energy storage and are therefore attractive for frequency stabilisation. They deliver high power at grid relevant frequencies of 120 Hz down to Hz, charge / discharge times of 8 ms 2 s, cycle life up to > 10 6 cycles (perhaps up to 15 years) and a round-trip efficiency of > 95 %. Challenges 10 times higher cost per kwh than Li-ion batteries. Unlike a battery, supercapacitor discharge voltage varies linearly with the charge contained in the system so that additional power electronics are required to ensure a steady output. Most grid storage demonstrators to date simply connect conventional supercapacitors in series to make large assemblies that are not optimised for the grid. Supercapacitors are considered safer than Liion batteries, but they use similar organic electrolytes and this presents a possible risk, especially in the event of a packaging leak or facility fire.

20 Examples of Supercapacitor Research The challenge for further penetration of supercapacitors in grid applications relies on enhancing their energy density, without compromising their defining advantage of high power, reducing cost, and ensuring a safe, long and low maintenance working life. Carbon from bio-derived sources offers potential to provide acceptable performance at a lower cost. The key is to develop high surface area bio-derived carbons that minimise costly high temperature processes (such as activation). Pseudocapacitors, notably RuO 2 and MnO x and some other transition metal oxides, have shown high specific capacitance, high energy density and long life in aqueous electrolytes. A strategy to mitigate the lower operating voltage of aqueous electrolytes is the use of dissimilar anode and cathode materials in asymmetric supercapacitors and, when combined with low cost materials, is likely a key feature of any future grid-relevant supercapacitor. Usually a carbon-based negative electrode is paired with a transition metal oxide-based positive electrode, so that the increased overpotentials at the electrodes delays the evolution of hydrogen and oxygen from decomposition of water, increasing the electrochemical stability window of the system. US based Aquion use a MnO 2 -based cathode, activated carbon anode, cotton-based separator and aqueous Na 2 SO 4 electrolyte. Performance is uncompetitive for traditional supercapacitor applications in mobile systems, but is aimed at the grid where 200 $ / kwh is targeted and systems at the 1 MWh scale are now being deployed.

21 Power to Gas

22 Large scale energy storage with hydrogen Over 400 billion m 3 of natural gas is currently stored underground worldwide. Hydrogen is already stored in salt caverns in the UK and USA. Hydrogen has over 200 times the volumetric energy storage density of pumped hydro and 50 times that of compressed air, for example. Taking into account the fact that a typical large salt cavern field has a volume of 8x10 6 m 3, this would provide a hydrogen energy storage capacity of 1.3 TWh per field. The use of salt cavern stores for hydrogen is considered essentially mature, with hydrogen stored at pressures up to 120 bar. Other potential hydrogen stores, such as depleted gas fields, rock caverns, aquifers and abandoned mine sites, require development to demonstrate that these could also be safely used. Hydrogen could also be generated and used locally, for hydrogen transport, or for hydrogen heating, through renewable electricity coupled with electrolysis.

23 Power to gas for grid applications Advantages Hydrogen offers the potential for very long terms energy storage at scale. This approach links the electricity network and the gas network. Green hydrogen has the potential to help decarbonise otherwise difficult sectors such as transport, industry and heat. New polymer electrolysers offer flexible response and can produce high pressure hydrogen. High temperature steam electrolysers offer very high electrical efficiency, and can recuperate high grade heat. Challenges The energy conversion efficiency of electricity to hydrogen for current electrolysers is around %, meaning that relatively low round trip efficiency of about 50% are obtained for electricity-hydrogen-electricity. The overall conversion efficiency of electricity to synthetic methane is estimated at around %. Electrolyser costs remain relatively high. Only alkaline electrolysers are a mature technology, though polymer electrolysers are being increasingly demonstrated. High temperature electrolysers remain in the technology development phase.

24 3D images of porous Ni-GDC SOEC cathode material, showing three phase boundary distribution Examples of power to gas research In order to make P2G attractive for grid storage applications, key challenges in three main areas of research need to be addressed: continued cost reduction of the electrolyser systems, extension of electrolyser lifetime and the development of more efficient and flexible electrolysers. This requires a full understanding of materials behaviour to be developed under realistic operating conditions. Very long term storage requires attention to be paid to the demonstration of hydrogen storage in salt caverns and in the integration of electrolysis with CO 2 to produce synthetic methane. Ni (with GDC) TPB TPB (with GDC)

25 UK Research Needs in Grid Scale Energy Storage Technologies; N Brandon et al, April 2016, from Energy SuperStore, Energy Storage for Low Carbon Grids and IMAGES, funded by EPSRC. Summary While storage technologies are being deployed on the grid, and products such as the Tesla Powerwall are becoming available, storage technology needs to continue to come down in cost, and lifetimes need to increase. Sodium ion batteries offer a cost reduction from lithium ion batteries, and no concern over materials supply. New chemistries and cell and stack designs offer the potential for significant cost reduction in flow batteries. Aqueous based supercapacitors, designed for grid scale applications, offer fast storage at much lower cost than current technologies. New battery measurement, interfacing & control methods are emerging that will reduce system cost and increase lifetime. Hydrogen from renewables offers truly long term storage and also supports the decarbonisation of transport and heat.