Working principle of photovoltaic energy storage system

Use the electricity from your photovoltaic system as well as possible. If there is a surplus of solar power, charge your battery storage and use the energy when your demand increases. Here you can find out everything you need to know about electricity storage.

What is electricity storage?

A power storage device is characterized by the fact that it can store generated electricity and release it again later. There are various ways of exactly how this happens, which is why many types of electricity storage exist.

From a technical point of view, electricity storage for PV electricity, also referred to as solar storage, is usually large accumulators. They store electrical energy in the form of chemical energy. They are also sometimes referred to as battery storage or a solar battery. This is not entirely correct because originally, only non-rechargeable stores of electrical energy were called batteries ("primary cells" or "primary elements"). In contrast, accumulators are rechargeable storage devices ("secondary cells" or "secondary elements"). In the meantime, however, these distinctions have become linguistically blurred. The most prominent example is the "car battery," which represents a lead-acid battery.

Electric storage systems have recently gained importance as a supplement to photovoltaic systems. On the one hand, users are striving for greater independence. On the other hand, using a solar storage tank is becoming more and more economical.

The feed-in tariff for solar power has been below the electricity price for years. Since then, it has been mathematically cheaper for operators of private photovoltaic systems to consume the solar power themselves instead of feeding it into the public grid. For a long time, however, the prices for electricity storage were too high for such an investment to be worthwhile.

As the installation figures show, the demand for electricity storage for private use has increased significantly in recent years. The market research institute EuPD Research spoke of 106,000 new photovoltaic home storage systems in 2020. Three years earlier, the addition amounted to 37,500 devices. This corresponds to almost tripling the market for battery storage. The main reasons for this trend are the desire for lower electricity bills and independence and significantly falling solar storage prices.

Batteries and accumulators have been used in all areas of life for a long time, from small power storage devices in digital end devices to large power storage devices for entire districts. It is not surprising that there are also a large number of rechargeable batteries that are used to store solar power.

Construction of a solar power storage system

In general, power storage devices do not consist of a single battery cell but of many connected cells. For the memory to work without problems, it needs a battery management system (BMS) that monitors the individual cells and ensures the charge level is even. If individual battery cells threaten to overheat, the BMS switches off the power storage. It also determines the loading strategy of the storage system.

Many current storage systems are characterized by a modular structure, meaning that the control unit with the battery management system is in a storage unit. The battery cells, i.e., the actual storage units, are housed separately. The advantages of the modular design are obvious. Within a certain framework, the control unit can be combined with any number of storage units and thus tailored to the user's needs. Individual modules can be replaced without replacing the entire power storage unit in a defect.

Some PV battery storage also has additional modules or components with which an emergency power function can be implemented.

A basic distinction is made between an AC and a DC-side installation for electricity storage systems. With DC-side integration, the solar storage tank is connected directly to the PV system and can directly store the direct current from the modules. With the AC-side integration, the power storage is located behind the inverter. The power inverter provides converted AC power, which is then converted back to DC by a battery inverter for storage.

Of course, the DC-side power storage cannot do without an inverter because household appliances require 220 V AC. For this conversion of stored direct current into alternating current, the storage accesses the PV inverter.

Pros and cons of AC and DC storage

Solar storage systems installed on the DC side are the better choice because two fewer conversions are required between the PV system, storage system, and consumers in the household, which positively affects system efficiency. DC storage does not require a battery inverter. As a rule, its installation also requires less space and effort.

A major disadvantage is that the battery storage has to be adapted to the performance of the PV system, which is why it tends to be more suitable for new photovoltaic systems. In addition, the PV inverter's power limits the system's power and solar storage since it has to serve both.

The great advantage of the battery storage integrated on the AC side is its flexibility. They are independent of the power of the PV system and the PV inverter and allow any storage capacity. This makes them particularly interesting for retrofitting an existing system. AC storage can also easily store grid power. This can be useful if very cheap tariffs are available.

Single or three-phase power storage - which is better?

Another difference is the grid connection of the storage system. In Germany, power up to 4.6 kW can be fed into the home network in a single phase. A three-phase system is recommended for higher outputs and is also necessary for more than 13.8 kW. However, both variants have no disadvantages in terms of regulation and billing. Irrespective of the mains connection of the storage tank, the energy flows in the solar storage tank is measured in three phases. For this purpose, the specialist company installs a small meter in the house distribution system.

This is how a PV power storage system works

The electricity generated in photovoltaic systems is initially used for current consumption. This means active electricity consumers such as a refrigerator or lighting are operated with the available electricity. However, if more electricity is available than required, the excess solar power flows into the storage tank, which is charged. Only when this is full is electricity fed into the grid.

Suppose the demand for electricity during the day or in the evening is higher than the amount of solar power produced by the solar powered generator. In that case, the battery storage makes the difference available - regardless of whether it is fully or only partially charged. Only when the stored solar power is no longer sufficient is additional main power drawn from the energy supplier. In this way, it is possible to cover a large part of the required amount of electricity, i.e., self-consumption, with the electricity generated by the photovoltaic system.

Solar energy

Advantages of a power storage

The main advantage of an electricity storage system in combination with a photovoltaic system is immediately apparent: photovoltaic systems produce solar power that must be used immediately. This is rarely effective as most electricity is generated during the day. During this time, however, the electricity requirement in the household is usually rather low. On the other hand, the demand increases significantly in the evening, i.e., when the solar power yield is low. With a battery storage system, you can use solar power not required during the day when you need it.

The storage thus increases your self-consumption and lowers your electricity costs. Because you no longer have to feed excess electricity into the grid to buy it again at a high price later. Instead, you use your electricity and thus make yourself less dependent on the energy supplier.

Characteristics of electricity storage

Many products challenge the battery storage comparison, not only for laypeople. To be able to compare individual models with each other, you should therefore know a few important terms:

Storage capacity

This parameter indicates how much electricity (in kWh) the solar storage system can absorb and deliver. An increase in storage capacity is usually associated with a higher price for storage. Additional investments only make sense if the storage capacity can be used fully.

Note: The storage capacity is given in kilowatt-hours (kWh). A power storage unit with a capacity of 6 kWh can therefore deliver an output of 1 kW for six hours. It is more common to design solar storage so that the household can be supplied with stored energy from evening to morning. Typical capacities for home storage are between 6 kWh and 10 kWh. If heat pumps or electric cars also have to be supplied, the battery storage must be dimensioned accordingly larger.

Depth of discharge

The depth of discharge indicates what percentage of the stored energy can be removed from the device. Most solar storage tanks need a residual charge to prevent damage, according to the German Solar Industry Association e. V. should never fall below the depth of discharge specified by the manufacturer for a power storage device, as this significantly reduces the battery's life.

Depths of discharge between 50% for lead-acid batteries and up to 100% for lithium-ion batteries are customary on the market. With a depth of discharge of 50%, the usable storage capacity is only half the nominal capacity.

System efficiency

Conversion losses occur during the storage process, i. H. part of the energy is converted into heat, for example. The overall efficiency indicates what proportion of the energy stored in the system is used. Fluctuations between 70% and 95% are customary in the market.

Please note that battery efficiency and overall efficiency are different figures. The overall efficiency does not only refer to the battery but to the entire storage system. Due to conversion losses, the overall efficiency is usually significantly lower than the battery efficiency and is kept secret by most providers.

Service life/number of cycles

The expected lifetime of power storage varies between 5 and 20 years, depending on the model and manufacturer. Also, pay attention to the manufacturer's performance guarantee, which, for example, guarantees at least 80% capacity of the battery module after 10 years.

The number of cycles is usually given to measure the service life, i.e., how often the memory can be loaded and unloaded when the total useful capacity is used. In addition to the number of cycles, the aging of the memory also plays a role, i.e., the chemical change in the idle state. This aging is determined by the system but also by the charging strategy.

Loading strategy

Like all rechargeable batteries, power storage devices are also affected by aging. This means the battery's capacity decreases with the number of charging and discharging processes. Chemical changes at the interfaces of the electrodes cause this. This aging is accelerated if the storage remains fully charged for a long time. Some manufacturers adopt the charging process so that the solar storage tank is only fully charged for as long as possible before the next discharging process begins. However, there is a risk that the memory is not fully charged, e.g., in the event of a sudden change in the weather. To avoid weather forecasts can be incorporated into the charging strategy.

Response times

Like any technical device, solar panel battery storage systems are "sluggish," i.e., it takes some time to react to a change. For example, if a consumer is switched on in the household, it takes a while for the battery to supply electricity. In the meantime, mains power is consumed. This mains current requirement is negligible with constant power demand, but large consumers, such as electric stoves, demand pulsating current. With such devices, a lower memory response time pays off.

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