Looking at E-mobility from the other end of the spectrum

Solar-charging of electric cars

5. December 2017 | Engineering Service

The environmental benefits of electric cars have been constantly criticized, whether it’s talk of notional charging using CO2-intensive brown-coal power or battery manufacturing which consumes a great deal of energy and raw materials. If the battery is charged solely from the German public grid, then its benefits over gasoline or diesel vehicles are minimal across the life cycle of a vehicle, taking into account battery manufacturing. However, the picture brightens if electric cars can be charged using renewable energies. The following discloses how solar power is becoming less and less expensive and how higher home consumption will improve the profitability of the photovoltaic system and what all this has to do with charging an electric car.

With the introduction of the Renewable Energy Law (EEG) in Germany in 2000, all solar power generated was fed into the public grid because the compensation was considerably higher than the price for household electricity. Since 2013, however, the point has been reached, for the first time, when it is more economic to cover home consumption with solar power and only feed excess solar power into the public grid (grid parity). Now, the higher the price for household electricity climbs in the future (and solar power continues to drop), the higher the cost benefits will be, due to the home consumption of the self-produced solar power.

Average PV price for one watt (peak) in conjunction with installed PV output.

Increase in Home Consumption Due to Stationary Power Stores

In private households, the individual load profile of home appliances cannot be shifted to a preferred area with more sunshine. That is why limited home consumption is an issue. However, if, in addition to the PV system, a stationary power storage system is used, self-consumption can be increased because now solar power can be stored even when the sun is not shining.

When the sun is shining at its peak, the battery is initially fully charged and the excess PV power is fed into the public grid. In the evening hours, if the PV system’s power declines, the necessary household electricity is drawn from the battery. If the battery is empty, electricity is drawn from the public grid.

Annual Power Consumption

According to statistics, a one-person household in Germany consumes 2,050 kWh electricity annually. Due to their lower range and typical drive profile, electric vehicles are more likely to replace gasoline-powered cars than diesel cars because diesel engines have higher annual mileage and are operated for longer single trips per day. The average mileage for gasoline-powered vehicles is 11,000 km per year. In practice, consumption in an average electric car can be presumed to be 18 kWh/100 km. Over the year, that means 1,980 kWh for one electric car, which corresponds roughly to a one-person household.

>> AN ELECTRIC VEHICLE CONSUMES ALMOST AS MUCH POWER ANNUALLY AS A ONE-PERSON HOUSEHOLD

Adjustment of the Electric Car’s Charging Capacity to the Solar Power Generated

There is a broad range of possible technical solutions for charging an electric car with solar power. Energy management systems frequently come with remote-controlled sockets that can easily switch the electric car on or off like a home appliance, depending on the solar in-feed. This often causes error messages in the vehicle and there is no option to obtain a flexible charge capacity. A better and nevertheless simple solution consists of a small solar cell with an adjustable voltage divider that delivers 0–10 V as a direct current input signal for a charge box (in cable control box = ICCB). The charging capacity can be gradually increased or decreased by modulating the pulse width in the square wave voltage (PWM signal) and can therefore easily track solar radiation. Through the charge box with type 2 output, both vehicles that have a type 1 as well as a type 2 connection can be charged using solar power. There is often a simple switch next to the wall box that can be used to switch between charging the vehicle with excess solar power and immediate charging.

Comparison of the cost trends between household electricity prices and the EEG compensation for PV systems with an installed output of less than 10 kW.

>> ENERGY MANAGEMENT SYSTEMS FREQUENTLY COME WITH REMOTE- CONTROLLED SOCKETS THAT CAN EASILY SWITCH THE ELECTRIC CAR ON OR OFF LIKE A HOME APPLIANCE, DEPENDING ON THE SOLAR IN-FEED

More precise – but often more expensive – are the home automation systems that can, for example, route the ongoing solar power generated, the power from the grid, or the power in-feed to the bus and generate direct current for the charger via an actuator. In addition, there are also complete systems that cover the entire product range, from the PV module and energy storage to inverter and the wall box in the garage. Smartphones and tablets can often be integrated for display and operational purposes. Weather forecasts are sometimes incorporated and the home energy management system can also be optimized according to certain parameters and, in part, on a self-learning basis.

Characteristic performance curve of the operation of a typical combined PV-storage system over one day.

Highly regulated solar in-feed according to EEG

Since the EEG 2012 was amended, the output of a PV system can be reduced or even shut off. That’s why it is necessary to procure a shut-down device. For small systems, this is unduly expensive. If you want to circumvent this, you have to limit the in-feed power to 70% of the maximum possible output right from the start.

While limiting output on east/west-facing roofs does not present a problem because both roof surfaces cannot simultaneously yield the same high performance, for south-facing roofs, it is important to promote high home consumption in the afternoon using an intelligent control system and connectible consumers so that the solar revenues above the 70% limit are not lost.

Aside from the technical limitations, there are several bureaucratic requirements. A PV system must be reported to the Federal Network Agency. If you want the tax authorities to reimburse you for the sales tax and you have waived the small business regulation, then you are further required to submit an advance sales tax notification monthly. In the process, the power consumed in the home is balanced and the sales tax recovered. For small PV systems, this time and effort is not to be underestimated.

Typical (not optimized for the grid) performance curve from a PV system with energy storage


Performace curve of PV system with energy storage and operations management optimized for the grid

Increase in “Solar Guerrilla” Solar Power Systems

A new movement called the solar guerrilla movement refers to the installation of single or numerous solar modules with micro-inverters on balconies, for example, and feeds the solar power backwards into the home grid through the outdoor sockets. There are already turnkey systems equipped with shockproof plugs that can be purchased in order to give renters without access to a roof, for example, the option of operating a solar power system with little or no installation time or effort. The purchase pays off due to a lower electricity bill over a period of eight to twelve years. Officially, a PV system must be reported to the Federal Network Agency and the utility company must also be informed. So that the old electricity meter does not move backward in extreme cases (more PV power generated than used in the home) and users are not prosecuted, the electricity counter has to be replaced by one that has a backstop.

Summary

In addition to selecting suitable cables and a wall box that is compatible with the electric car, there is an increasing demand from customers to generate their own electricity for charging the electric cars. Selecting suitable components (PV module, energy storage system, inverter, and intelligent wall box) is a very complex process and frequently pushes handymen and general contractors to their limits. The more previous knowledge that private or commercial customers have, the more the finished product will meet their needs and reward them with the satisfaction of producing their own power to meet their mobility needs.

EVA Fahrzeugtechnik
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