If the total installed photovoltaic capacity increases to a certain order of magnitude, there will be significant surpluses in places due to solar power generation. These would be difficult to manage without ambitious expansion of the electricity grid.

To make use of the surpluses and prevent shutdown of the PV system, storage capacity must be increased. Centralised storage systems such as pumped-storage power plants will only be able to cover part of this in the foreseeable future. By contrast, the potential of decentralised storage solutions can be tapped in the short term.

With a storage system, part of the self-generated solar power can be temporarily stored during the day for delayed consumption in the evening and at night. If the photovoltaic system generates more electricity than is needed in the building at the same time, the energy flows into the storage system. If more power is required than the PV installation can supply, one’s own electricity can be used by deferred discharge of the storage system.

Lithium batteries have become established on the market. They are characterised by high efficiency, high energy density and a comparatively long service life even when used intensively. The storage capacity can also be used in continuous operation with no major signs of battery wear.

In practice, storage systems are often installed that are too large, causing the battery’s state of charge to fluctuate between half full and full. The unused capacity unnecessarily costs money and wastes the raw materials and resources used to produce the storage system. Benchmark for calculating storage size: approx. 1 kilowatt hour of battery capacity per 1,000 kWh of annual electricity consumption.

The following specialist literature provides an overview:

• pv magazine – market overview (in German)
• HTW Berlin – storage system inspection (in German)

Battery storage systems have a service life of 10 to 15 years (in comparison: solar modules have 20 to 30 years). This is due to chemical processes in the battery cells. The ageing of the materials leads to a slowly decreasing storage capacity over time. This process accelerates towards the end of its service life.

Based on weather forecasts and previous consumption values, an intelligent charge control system supports battery-friendly charging and discharging and thus also has a positive effect on the service life of the battery. In many cases, an intelligent charge control system is already integrated into the battery storage system. Another option is external controls via an energy management system. This system centrally controls all large generation and consumption systems (such as PV systems, electric car charging stations, heat pumps, etc.).

From an economic point of view, a photovoltaic system with a storage system is worthwhile if the costs of generating and storing electricity are lower than the electricity price of the respective energy supplier. The main advantage of a storage system is that it increases self-consumption and self-sufficiency.

With the ability of bidirectional charging (electricity flowing in both directions), electric vehicles could be used in the future as a kind of large ‘power bank on wheels’ and as storage systems. In doing so, they contribute to grid reliability and are an important building block for the energy transition. In practice, electric vehicles are first integrated into the property’s electricity grid (for example, in combination with a PV system and a heat pump) and intelligently networked (vehicle-to-building). The second step involves integration into the general electricity grid (vehicle-to-grid). Intelligent charging management ensures that there is always enough energy available for electric vehicles themselves.

Some manufacturers have started to sell technically sophisticated bidirectional charging stations with CHAdeMO connectors. These systems are still in their infancy in Europe, but could become standard in the medium term. In Japan, bidirectional charging technology has been mandatory for every electric vehicle for years.

The term ‘thermal energy storage’ refers to both heat and cold storage. Heat and cold storage systems have an opposing operating principle. While a heat store absorbs energy, the cold store emits energy in the form of heat. If the stored energy is below the ambient temperature level, this is referred to as a cold storage system. A temperature above the ambient temperature, on the other hand, is heat storage.

In addition to storing thermal energy, the core aim of thermal energy storage is to break the link between when the heat or cold is generated and when it is used.

A thermal energy storage system can be characterised primarily by the storage temperature, the specific heat storage capacity and the efficiency. In addition, the specific investment costs are an important parameter for assessing profitability. However, they are highly dependent on different framework conditions (storage volume, storage medium, etc.) and location conditions. In thermochemical storage systems, heat storage is almost loss-free.

These are currently being used on more of an isolated basis for individual buildings, while thermochemical storage systems are still in the development phase. Both storage technologies have great development potential due to their high efficiency, high operating temperatures and high storage densities. With latent storage, an efficiency of 75 to 90 per cent is achieved.

These represent an established and cost-effective technology and are often used for solar local heating networks or low-temperature heating networks in new-build neighbourhoods. The large sensible water storage systems (hot water, gravel water, aquifer and geothermal probe storage) can generally be operated with a water temperature of 30°C to 95°C. The efficiency of sensible heat storage is between 47 and 75 per cent.

Cost-effectiveness is a decisive factor in the selection and implementation of storage technologies. Cost-effectiveness can be assessed on the basis of the specific investment costs.

Despite their high efficiency and high energy density, thermochemical storage systems require significantly higher investment costs than sensible and latent storage systems, depending on the storage material. Due to the early development of the technology, the use of thermochemical storage is associated with a high economic risk and unanswered scientific issues.

Relatively high heat storage capacity also leads to higher investment costs for latent storage systems than for sensible heat storage systems. Investment costs for latent storage systems increase significantly with high capacities.

Compared to latent and thermochemical heat storage, the use of sensible storage systems is at an advanced stage of development and has significantly lower investment costs, which means that very high storage capacities can be opened up for whole neighbourhoods in a cost-effective manner. Sensible heat storage systems at district level are currently being piloted. Investment costs are highly dependent on local conditions.

‘Cross-sector’ describes the interconnection of different sectors, such as electricity, heat and mobility.

Energy storage by means of such systems can be carried out with or without energy subsequently being reconverted and fed back into the grid. With ‘Power-to-X,’ surplus renewable electricity can be used to produce energy sources (e.g. for heat production, later reconversion, mobility) or commodity chemicals for fertilisers. Numerous countries have high hopes for the future potential of this technology in their climate policies.

Source:

• ‘Verbraucherzentrale Nordrhein-Westfalen’ consumer organisation. ‘Is battery storage for photovoltaic systems worth it?’ (in German)
• Dena study ‘Thermal energy storage systems for neighbourhoods’ – 09/2021
• Technical literature ‘Energy storage systems – demand, technologies, integration, 2014’
• Swiss eMobility ‘Bidirectional charging’ (in German)
• Paul Scherrer Institute. White paper ‘Power-to-X: outlooks in Switzerland’ (in German)