E-mobility solutions in photovoltaics
As already known, the infeed remuneration in Germany is not a constant in the market and occasionally – yet without fail – leads to frustration among fitters and project planners. On top of this, electricity prices continually seem to rise. No wonder then that the focus is increasingly shifting from surplus remuneration to self-consumption optimisation. For a number of years now, the use of battery storage units has become established in photovoltaic systems, with the aim of utilising generated electricity as effectively as possible oneself. Yet the options for self-consumption optimisation are numerous and attracting increasing attention. Although still a niche product, it could help shape the growth in e-mobility in the years ahead: a charging station – or more colloquially wall box – for your own home.
The basic principle behind self-consumption optimisation
Charging stations have been around for a long time – what’s new is their integration in photovoltaic systems and the advantages associated with this. To make sure primary solar electricity is used for charging, two preconditions basically need to be met:
- The charging station must be “intelligent” or enable control.
- The surplus from the generation system must be known.
The setup is the same as for the battery storage units mentioned above: the current flows are logged at the network infeed point and the battery or consumer activated upon surplus.
Conventional battery system:
System with charging station:
As you can see, a communication connection to the PV system is indispensable to enable optimum use of the solar electricity. The energy balance on a typical day in an existing system is outlined below. The charging capacity of the wall box is shown in red. The control of the charging capacity and “simulation” of the generation curve are clearly discernible.
Charging station control:
Already profitable after a short time
Rising electricity prices and falling system costs for PV will make the investment ever more attractive in future. How soon a wall box pays for itself depends on may factors such as charging time, size of the battery, size of the PV system – so a straightforward answer is not possible. However, a typical calculation reveals that the investment can pay off after just 1-2 years.
- Investment costs for charging column: €1500
- Electricity generation costs: 8 cent/kWh
- Daily battery charging: 15 kWh
- Working price for electricity procurement: 25 cent/kWh
- PV system adequately dimensioned so that enough surplus is available.
Difference between electricity costs and solar energy: 25 cent – 8 cent = 17 cent
Saving daily: 15 kWh x 17 cent/kWh = €2.55
Saving yearly: €2.55 x 365 days = €930.75
Amortisation time: €1500 / €930.75 = 1.61 years
A hundred percent coverage with solar electricity is rarely possible in practice, but this shows that an investment can pay off quickly.
Wall box versus sockets
Maybe you’re asking yourself why a charging station should be installed when standard sockets are usually available in the garage. The reason for this lies in the very limited charging capacity of household sockets, also called protective earth sockets. These are designed for maximum 3.7kW (230V, 16A), but often cannot withstand a continuous load at maximum power, resulting in numerous burnt sockets in practice. Burnt sockets and cable fires can pose an enormous safety risk if the system is not fused either. Plus it follows logically that low charging capacities entail longer charging times. The theoretical charging time t can be calculated by dividing the capacity C by the charging capacity P
t = C/P
Charging stations can charge up to 22kW, occasionally even up to 43kW. A comparison of the charging times with a 20kWh and 40kWh battery:
It should be noted here that the maximum charging capacity is also limited by the vehicle, i.e. it is not possible to carry out a fast charge with every e-car available on the market. Before choosing the appropriate charging station, the technical restrictions should therefore be clarified first.
Current plug types
The charging capacities of the wall boxes also give rise to new challenges when plugging in an electric car. Among the large variety of plugs, a few have become established as standard.
This plug is a single-phase AC plug, which achieves a charging power of up to 7.4 kW (230V, 32A). The type 1 plug plays an important role predominantly in the USA and Asia. This plug type was encountered sporadically in Europe in the past before the type 2 plug was agreed as standard.
This is the standard plug for Europe (from 2017 as European standard for all new vehicle models) and was developed in Germany. The type 2 plug is a three-phase plug, in which charging capacities up to 22kW (400 V, 32 A) in private use and 43 kW (400 V, 63 A) in public charging stations are common. Most “wall boxes” and charging stations are equipped with type 2 plugs.
Charging with the CCS (Combined Charging System) plug works on the basis of the type 2 plug. More precisely, it supplements the type 2 plug with two additional power contacts thanks to a quick-charge function and supports both AC as well as DC charging. CCS plugs currently have a charging capacity of approx. 50-100 kW.
The reconstruction and loan institute KfW (Kreditanstalt für Wiederaufbau) funds charging stations. Put more precisely, it supports “environmental protection measures by commercial enterprises and freelancers”. Details such as loan amount, term and specific interest rate are tailored individually to the relevant company. This funding is only available to domestic and foreign companies of any size, freelancers, public-private partnership models and companies that provide services for third parties as contractors.
The market for e-mobility will continue to grow.
It is clearly evident that manufacturers are responding to the trend towards self-consumption optimisation, while private individuals wish to be less dependent on the infeed remuneration. PV-coupled charging stations from Mennekes are already available and can be used in conjunction with SMA inverters. SolarEdge is also set to market a hybrid unit comprising inverter and charging station. Wall boxes are the next step towards making renewable energies economically attractive not only for idealists but also the public at large.