Let’s take a look at energy storage technology, with a focus on the benefits offered by small-scale storage, which is increasingly accompanying bids for micro photovoltaic installations.
The electricity produced by photovoltaics can be stored in a number of ways. Some of these are suitable for small installations, while others only make sense for large-scale production. The most popular of these is conversion to electrochemical energy. This is the principle behind the currently popular lead-acid, nickel-cadmium (NiCd), nickel-metal-hydride (NiMH) and lithium-ion batteries.
The way this works is quite simple - a photovoltaic installation generates electricity which is sent to accumulators (less frequently batteries), which retain it in the form of a chemical substance. Once the substance is converted, it becomes usable electricity again. Batteries store direct current at a voltage, i.e. a 12 V battery. In order to use the energy stored in batteries, it is necessary to connect to an electronic device, such as an inverter, that converts direct current into alternating current.
In order to fully answer this question, we need to analyse the different purposes for which energy storage is used.
This is the primary and most cost-effective way of using photovoltaic energy. Instead of selling the solar energy, it is used on site as much as possible. The profit here is the difference between the purchase price of energy from the grid (cost of active energy + distribution charges) and the sale price of PV energy.
This option will be available in Poland from July 2024. Some energy storage systems can sell energy to the grid in the evening when it is very expensive to consume. This is particularly beneficial for companies that do not need large amounts of energy in the evenings and who have a surplus from daytime PV production. Profit is made from the difference in the price of energy in the evening and the price of energy during the day.
Optionally, storage facilities can be used to recharge with energy bought at low prices and use it at times when it is more expensive. The profit, again, is the difference in energy price depending on the time of day.
Energy trading companies pay for the opportunity to dispose of part of the energy from your storage, which allows them to use energy market mechanisms. In the near future, this will also include, as the example of Germany shows, the possibility for these companies to earn money from flexibility services for the power grid.
In addition to previously mentioned capabilities, energy storage facilities also benefit many other aspects of the business operations. They are great as sources of emergency power, solve the problem of photovoltaics shutting down due to over-voltage on the grid and support the charging of electric cars.
Businesses may also find the benefits of energy storage particularly interesting. Here are a few examples:
Conversion to electrochemical energy is just one way of storing energy. If you produce a lot of it, it is worth exploring other storage solutions - they may prove to be more cost-effective for particular types of activity.
There are two ways of converting to mechanical energy, namely the use of compressed air and pumped storage power plants. Both only make sense for very large amounts of energy. The former is a low efficiency method that won’t make sense for small amounts of energy. The latter, while attractive because of its very high efficiency (in the range of 65-85%), requires huge reservoirs.
The most popular chemical storage options are hydrogen and methane production. The advantage of energy storage in the form of hydrogen is that it can be stored for long periods and has a high energy density. Methane is easier to produce but has the disadvantage of energy losses during the production process, which result in a total energy recovery of less than 38%. Methane generation is attractive for storing excess energy generated by large photovoltaic and wind power plants.
If a business decides to purchase storage using electrochemical processes, it is important to consider carefully what those energy storage needs are.
As a rule, companies have a high self-consumption. With energy storage they can reach a state where all the energy from photovoltaics stays inside the company. In addition to this, batteries make it easier to control the power drawn from the grid, which is of considerable importance, and can also ensure continuity of production in the event of grid failure.
Plant owners and other larger energy consumers should consider flow storage due to its low price and slow degeneration.
An interesting energy storage alternative is hybrid storage which combines flow storage and lithium-ion storage. Flow storage slowly charges throughout the day, while the lithium-ion storage only switches on during peak production hours (so-called peak shaving).
For farms, it is very important to increase self-consumption and ensure a continuous supply of electricity. Refrigeration equipment is a good example of this. Any energy storage unit with an emergency power supply function will fulfil these functions.
When buying a photovoltaic installation at the same time as energy storage, which is the best solution from an economic point of view, the lifetime of the energy storage device becomes important.
Assuming that the solar panels will last around thirty years, it is only natural to expect that the storage unit would last for the same length of time. Unfortunately, that’s not the case. Although the lifespan of energy storage unites depends on many factors, such as how it is used, how well it is cared for and its long-term operating characteristics, it is assumed in most sources to be an average of 15 years - or around 6,000 full charge cycles.
These are a type of high-capacity electrolytic capacitor. They accumulate charge on a double electrical layer, which is formed at the boundary between the electrolyte and the electrode. The use of carbon nanotubes allows for a large surface area, resulting in an increase in capacitance of the capacitors.
The advantage of supercapacitors over batteries is their higher energy density. This means that they are able to give off energy with a high-power output. Furthermore, a huge advantage is the low decrease in performance characteristics and their long service life (up to 20 years of operation).
There are many additional storage solutions. Those already mentioned in this article are currently in use or being intensively developed. In addition to these, there are also solutions that have found their niche (e.g. conversion to kinetic energy in hybrid cars) but have little chance of expansion. And there are those that may begin to dominate in the future (such as the use of thermal processes in the increasingly popular heat pumps). Such technologies include:
Magnetic fields are the clear leader when it comes to storage time and system efficiency. The magnetic field created by the current in a superconductor lasts for an infinitely long time and, consequently, energy can also be stored in it for any length of time. The efficiency of such a system is in the order of 95%. Unfortunately, the need to use a superconductor involves very high cooling costs for the system. To make matters worse, the technology does not provide the ability to store large amounts of energy.
In a thermal process, energy can be stored by cooling the air to -195°C which condenses it. The volume of air drops by a thousand times and storage becomes much simpler than storing compressed hydrogen. Using waste heat from power stations can increase the efficiency of the process by up to 70%.
This requires two tanks with a large thermal capacity connected by a pump. When demand is low, heat is pumped into the hot tank. When there is a higher demand, the pump is switched off and instead turbines are run to generate electricity through the temperature difference. The UK, where this technology is being developed, predicts that it could become a competitor to pumped storage power stations, with costs of around $35/MWh and energy recovery efficiencies of 72-80%. It is easy to imagine that as technology advances, reservoirs will become smaller and smaller and we will be able to afford our own mini pumped storage power stations, if not in our homes, then certainly on a medium to large scale.
In conclusion, there are already many benefits to having energy storage. When dynamic tariffs are introduced, their profitability will only increase. This is because they allow both increased self-consumption of self-generated energy and the sale of surpluses to the grid during periods of high prices.
In addition, energy storage facilities enable the use of cheap electricity at times when market prices are higher and offer the possibility of making stored energy available to energy trading companies for a fee.
Finally, energy storage solutions also provide a source of emergency power, ensuring independence and continuity of energy supply in the event of an emergency. By storing energy at peak production times by renewable sources, they significantly reduce their disconnections from the grid, while increasing the stability of the nationwide power grid.
All these benefits mean that energy storage plays an increasingly important role in a sustainable and efficient energy system.