Power generation and grid services
A photovoltaic installation (PV system) that feeds the entirety of the power it generates into the grid comprises the following essential components:
- PV generator (solar modules)
- substructure (mounting frame)
- direct current cabling (DC)
- generator junction box (GJB)
- alternating current cabling (AC)
- monitoring system
- feed-in meter
- grid connection point
The way in which PV systems are structured is relatively simple and universal. In principle, small PV installations with kilowatt-range outputs are therefore constructed in precisely the same way as large multi-megawatt solar power plants, and this has helped photovoltaics to gain a global foothold within just a small number of years. However, photovoltaics would be able to “penetrate” national power supply systems even more rapidly if uniform international standards were established.
Standardization would simplify installation and help both to avoid errors and improve operational reliability. This not only applies to plug connectors (see chapter “Cables and Connectors”), but to all aspects of systems engineering. Component costs would also fall, as it would be possible to manufacture in larger quantities.
The market still comprises two large module families: Modules with crystalline silicon (cSi) cells are dominant, though modules that employ the various thin-film technologies are also holding their ground. When planning a system, the unique characteristics of the different module types need to be considered along with the technical attributes of the inverters, where numerous variants are also available and the range of functions grows year on year. Such diversity calls for optimized component matching. The primary goal here is to ensure the highest possible system yield and a long plant lifespan.
In order for photovoltaics to become a mainstay of the power supply system, its contribution must extend beyond merely feeding the grid. For example, large-scale PV systems now help to stabilize power supply by providing grid services. When needed, they reduce the power output that is fed into the grid and provide both static and dynamic grid support.
The central interface for these services is the inverter, which besides tracking the maximum power point (MPP) and converting DC to AC is increasingly required to perform energy management tasks. This particularly applies to distributed PV installations, where a portion of the electricity is fed into the household network. Diminishing feed-in tariffs (FIT) may make on-site consumption a more economically viable solution in certain circumstances. Depending on supply and demand, the inverter feeds solar power either into the household network, a power storage unit or the public grid. The different options for utilizing the solar power must be assessed to find the most efficient solution for each situation.
Centralized and distributed storage systems
In certain German regions, so many PV systems have now been installed that on sunny days there are periods when more power is generated than is being consumed. Many of these systems are located great distances from large consumption centers, meaning that grid expansion is often not the best solution. The most sensible approach is essentially the local consumption of decentrally generated solar power.
The progressive expansion of photovoltaics will also cause regional surpluses of solar power to grow. To enable this energy to be exploited in future, distributed storage systems must initially be developed, followed later by centralized systems.
Just like solar power generation, growth in energy storage is “from the bottom up”, hence it starts with the smallest systems and gradually develops into bigger and bigger systems. At a basic level, storage systems only capture the power generated on roofs of individual homes. A storage capacity of merely a few kilowatt hours (kWh) is sufficient here, and can be provided by conventional lead- acid batteries. Lithium-ion batteries are seeing wider use, and for large-scale storage systems with capacities over several hundred kilowatt hours, redox flow batteries are also expected to come into play. This development is still in its infancy, however, and it is not yet clear which systems will prevail within the different power generation capacity levels. From a current perspective, the best prospects would be afforded by a mixture of distributed, short-term storage systems and large seasonal storage systems that can balance supply and demand in the long term and are able to store surplus solar power for several months.
Given the high costs associated with electricity storage, expanding the transmission networks may be one option for utilizing surplus regional power more wisely. Although this can be cheaper, there are limiting factors. Overhead power lines are not always welcomed and at times cabling sections need to be laid underground in order to conserve the landscape, causing costs to escalate. Expanding the grid does not create conflict with the development of storage systems, rather both systems complement one another in striving to utilize surplus solar power as efficiently as possible.
Off the grid
Photovoltaics provides excellent services in the public grid, but must compete here with other power generators and is consequently subject to pricing pressures. Solar power is most valuable in locations where there is no existing power grid and electricity is produced in small stand-alone grids using diesel generators. This is especially true of locations in the earth’s sun belt. Here, photovoltaics is able to fully or partially supplant the electricity produced from diesel generators, not only saving on expensive fuel but also making a direct contribution to climate protection.
Such off-grid PV systems are able to generate power relatively cheaply, as supplying an on-site system generally costs far less than introducing long cables for connection to far distant grids. This is resulting in the construction of ever more PV island systems across the globe, particularly in sparsely populated and less technically developed regions in Asia and Africa. Between on-grid and off-grid systems we find those PV installations that operate in parallel to the grid and provide support for weak public grids. On-grid and off-grid systems are growing together. Photovoltaics is becoming universal.