Wednesday, 29 September 2021

Part 2 - Energy Storage

In Part 1 - The Role of Renewable Energy, I discussed various options for low carbon, or carbon free, energy generation. I concluded that, unless grid-scale energy storage is available, the variability of the most rapidly expanding renewable energy sources, i.e. wind and solar PV, limits their penetration into electricity generation and distribution systems. So what options are available for large scale energy storage?

There are many ways of storing and recovering energy. For electricity the one we are most familiar with is the use of batteries.

Battery storage
The following graphic, which plots energy density in MJoules/litre against MJoules/kg, puts into context the energy density of Lithium Ion and Zinc-Air batteries relative to other substances. The significance of energy density is that gasoline, for example, effectively stores about 50 times more energy per kg than lithium ion batteries and so is a much more portable energy source. This is important for powering aircraft and heavy vehicles.

As a result of its use in electric vehicles, battery storage technology is developing and getting cheaper; but currently batteries are still expensive, have low energy densities and a limited life. In my view both energy density and longevity would need to improve significantly before the use of batteries to smooth out variable production from renewables on time scales, of say 4 to 12 hours or more, would become economic on a grid scale. There are, nonetheless, many battery storage schemes planned or under construction in the US and internationally.

The planned UK scheme at London Gateway, Thurrock, Essex has a peak output of 320MW and a storage capacity of 640MWh. Thus, if the batteries can tolerate 100% discharges, it has a theoretical run time at maximum output of 2 hours but it’s difficult to know what proportion of connected demand this represents.

Compare this to the proposed 3.6GW Dogger Bank Wind Farm in the North Sea, which is being developed in three phases of 1.2GW each, and it’s clear that Battery Storage projects are a long way from acting as medium term backups to Wind Power Generation. (A Gigawatt is 1,000 Megawats).

There is also a 185 MWh battery installation in operation, the Hornsdale Power Reserve, in South Australia, which has a full output rated at 150 MW. This installation is used for frequency and voltage stabilisation and is very fast acting, being able to put power into the grid in milliseconds, and thus smooth out shocks and transients caused by generation blips and load switching. Using the equipment to support electricity supplies in this way has generated significant savings by driving down the cost of providing fast despatchable power, which is otherwise expensive due to the need to run fossil fuel generators in standby mode. Australia’s electricity supply is heavily dependent on coal fired power stations and it remains to be seen whether the use of renewables like wind or solar, combined with battery storage, can be competitive or even politically palatable.

What can be confidently said is that battery storage is going to become more and more important in supporting supply grids with a high penetration of variable renewables in the future.

Energy storage in molten salt
By heating up a salt until it’s molten, energy can be stored in tanks of the hot liquid salt for later recovery by power generation systems. This has been done on the full scale in several solar power plants using directed mirrors in the USA, Spain, and China.

Andasol solar power plant, Spain

It’s also proposed as an adjunct to high temperature reactors such as the Moltex SSR with GridReserve® and the TerraPower Natrium reactor, so that the stored heat can be used to run generators and provide peak power outputs when required.

Other types of energy storage
Other energy storage systems are being proposed and investigated, and some may be promising in the long term, but most are not yet proven to be economically viable or available for widespread deployment. Some of them are discussed below.

Generation of hydrogen
An interesting example is the proposal to use surplus energy generated by wind and solar, when it’s not required by the grid, to produce hydrogen by electrolysis which can be compressed and stored until it’s used: either to blend into natural gas distribution networks, or to power vehicles and aircraft, or to generate electricity during periods of low output from renewables.

The disadvantage with these concepts is their overall efficiency. For example the efficiency of electrolysis is currently 60-70% and the ongoing conversion back into electricity is 40-60% (depending on the method used). Thus the overall efficiency of the round trip is 24-56%, which is much lower than batteries, and there is also the energy cost of compressing the gas for storage but, providing there are periods of overcapacity of installed renewable electricity generation, the electricity used is effectively free.

Work is ongoing to establish the overall economic viability of hydrogen generation and storage.

Some boiler manufacturers like the UK company Worcester-Bosch are conducting long term research to determine the viability of using hydrogen blended with natural gas in a domestic setting. They have found that blends of up to 20% hydrogen require only minor changes to natural gas boilers.

Hydrogen can cause embrittlement of metals and some types of steel used for high pressure gas distribution pipelines may need to be lined or replaced depending on the steel used. Pipelines made from cast iron, polyethylene and elastomeric polymers are unaffected and so the operators of domestic networks are not at risk of major expenditure.

There are also other ways of generating hydrogen using high temperature chemical reactions that fit very well with new nuclear technologies.

Liquid air energy storage
A further method which is getting serious consideration and development is the use of liquid air as an energy storage medium. Similarly to the production of hydrogen, electricity production from wind and solar would be used to liquefy air at atmospheric pressure and when required it can be evaporated under pressure to drive generators.

A 5MW/15MWh demonstration plant, constructed by Highview Power, has been in operation since 2018 at Pilsworth, Bury, UK. Construction has started on an £85m project at Trafford Energy Park, Manchester, UK, which is due for completion in 2022 and will store 250 MWh of energy. The round trip efficiency of such systems is low at around 25% but this can be enhanced, with the use of a cold store, to 50%. If the facility is located near a source of waste heat like an industrial process or thermal power generation system, the efficiency can be further enhanced, and Highview Power claims 70%, without the need to augment the capital cost with heat recovery systems.

Electric vehicles
Another idea is that, when required, electricity could be fed back into the grid from the batteries in electric vehicles: and therefore at some future date a large battery storage capacity would be available to smooth out shortfalls between generating capacity and demand. Although this Vehicle to Grid concept is now being marketed by Nissan and Ovo, it's unclear how it would be managed on a large scale, but it would probably need an infrastructure that had connection points at the majority of parking places; as well as some form of smart metering that credited the vehicle owner if stored electricity, that had already been paid for, was drawn out of the vehicle's batteries. These connection points, or the vehicles themselves, would also need to be equipped with inverters capable of converting DC battery current into AC power and synchronizing it with the grid. This concept has the attractive characteristic that the extra cost of adding these facilities to the charging infrastructure or the vehicles themselves is marginal.

The transition to electric vehicles, which is happening at a surprisingly fast rate, will make a contribution to the reduction in greenhouse gases from burning hydrocarbon fuels. Even if the electricity to charge vehicle batteries comes from gas-fired power stations there will be some efficiency savings and hence overall less carbon dioxide and nitrogen oxides will be discharged to the atmosphere.

But the requirement to charge an ever growing fleet of electric vehicles will put an increasing burden on the electricity supply network. So, without making any allowances for the higher efficiency of electricity versus gasoline when powering vehicles, for the UK I calculate this will require 7 new power stations to be built each year over the next twenty years.

Total number of litres of road fuel in UK per year    45,000,000,000 l/yr
US EPA says 1 US gallon gasoline is 33.7 kWh        34
1 US gallon is 3.785 litres -                                   3.785l /US gallon
so to replace all road fuel would need                    400,660,501,982 kWh/yr
or per day                                                           1,097,700,005 kWh/d
or an average instantaneous demand of                 45,737,500 kW or 45,737 MW
allow double for peak load                                     91,474 MW
Each medium sized power station gives                  700 MW
so it would need                                                   131 new power stations 
or over 20 years -                                                 
per year

If anyone feels like checking this calculation they’re welcome!

Pumped hydro
One system that has been proved to be viable for energy storage is pumped hydroelectricity, which has had plants in operation for decades. They require two large lakes, one several hundreds of metres above the other.

Water is pumped up when electricity costs are low and released through turbines to generate electricity when required. Due to pump/turbine efficiencies and the two way conversion, the electricity recovered is only about 70-80% of the electricity input. The geographical opportunities for such installations are few, and so far they have only been used for providing flexible power for relatively short durations at peak times when the feed-in tariff for electricity is high.

An example is Dinorwig power station in North Wales which was started in 1974 and completed in 1984.

There are ten such schemes under construction in Europe totalling 1,339MW of capacity. To put this into context, these schemes would add less than 0.1% of the total energy consumption of the 28 EU countries in 2015 as storage capacity. (This total was 12,609 TWh, equivalent to a continuous consumption of 1.44TW. A Terawatt TW is 1 million Megawatts).

Backup power
Backup power production capacity or energy storage is therefore necessary to supply the demand for energy when wind and solar can’t and, because it takes time to bring generating capacity on line, some backup systems have to be kept running even when the demand is being satisfied by renewables.

The German experiment
Under the German EnergieWende, in which it’s intended to move away from fossil fuels and nuclear power to a low-carbon energy economy, up to 2014 they have been running lignite and hard coal fired power plants to replace the energy generated by nuclear plants which have been taken out of service, and to provide backup to wind and solar. During these years the consumption of gas has also been reduced: so the increase in energy from renewable sources has been offset by the reduction in the least carbon dioxide producing sources i.e. nuclear and gas.

The following graphic shows German energy consumption updated to 2019 and corrected for efficiency. 

This means that the vertical axis depicts the amount of primary energy required to generate the indicated amount of TeraWatt hours of electricity. 
Since 2014 the proportion of coal and nuclear electricity generation has decreased, that of gas and oil has stayed the same, whilst wind power has on the other hand increased. So far so good, but it’s clear that the total output from renewables has a long way to go before it represents a significant proportion of total generation capacity. Furthermore if electricity production is ever to be carbon free then the consumption of oil, coal and gas needs to be reduced to zero.

Coal is the most polluting fuel
Coal, and particularly lignite, is the most polluting of all the fuel options since, due to lower efficiencies, legacy plants produce more carbon dioxide per MWh than other fossil fuels and release, into the environment pollutants: including particulates; sulphur and nitrogen oxides; and ash. Ash contains uranium and is 100 times more radioactive than nuclear waste. It also contains heavy metals which will never decay and become less polluting. This ash is typically dumped or stockpiled with minimal control and has caused serious ash-slides necessitating dangerous clean up work

Air pollution from burning coal is even more serious because it travels further. If, like me you are old enough, you may remember the acid rain arising from UK coal burning power stations that acidified Scandinavian lakes in the 1980's. 

The horrific air pollution in China also dramatically illustrates the results of burning coal and is responsible for many premature deaths.

Germany has passed a law intended to phase out coal fired power stations by 2038.  This will commit Germany to closing all of its coal and lignite fired power stations, but it will clearly be very difficult to phase out both nuclear and coal fired generation, which together represent more than 50% of current capacity, and replace them with intermittent renewables. It’s not clear how Germany intends to fill the gap that it appears likely to create without a change of policy but it will probably be plugged by burning Russian gas.
In effect the German EnergieWende amounts to an experiment on a national scale.

You can be paid for using electricity!
Another effect of relying more and more on renewables has occurred in Germany on some sunny and windy days. Because German law forces their grid operators to accept renewable energy in preference to that from fossil fuels, and electricity production from fossil fuels cannot easily be ramped down, on occasions the price of electricity has become negative in response to an over-supply, meaning that commercial consumers are being paid to burn more electricity!

But Germany is not alone in this situation. In the UK, where there is a free market for electricity, there have been whole weeks when the day-ahead wholesale price of electricity was negative and on 21st May 2020 it fell to an average over 24 hours of minus £9.92/MWh.

It’s clear that this is a ludicrous waste of an otherwise precious resource that results in some measure from subsidies and, apart from providing windfalls to large industrial users who are able to ramp up their energy consumption, it has no advantages. This energy should be used for other purposes which are viable without continuous power, such as, perhaps, generating hydrogen for storage.

Renewable power alone is not the exclusive answer
My conclusion is that, over any specific 24 hour period, renewables can only satisfy a proportion of the total energy demand on any supply grid and a mix of different generation sources and storage types is necessary,

The next article, Part 3 Nuclear Power and the Reduction of Carbon Dioxide Emissions, advocates the use of nuclear power to secure the base load when wind and solar can’t. 
To enable the reader to contrast the technology of the current fleet of 2nd generation nuclear reactors constructed in the 1960’s and 70’s with the more recent and safer designs, that are explained in part 4 of this series, the next article  goes on to describe what second generation nuclear reactors contain and how they work.


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