When installing a photovoltaic system, our customer Mr. Nißle, a mechanical engineering graduate from Lower Saxony, asked himself to what extent self-consumption could be increased. The photovoltaic system, which delivers almost 20 kWp in total, should use part of the photovoltaic electricity generated by thermal coupling with the heating system.

A classic heat pump system for the well-insulated house seemed unsuitable because it is oversized due to the high costs for the heat pump system. The previous heating system is only used during the heating period and even here the energy requirement was very low with the existing gas boiler at 6,000 kWh per year. A heat pump would not have been economical even with funding.

After a short search on the internet, the decision quickly went in favour for an alternative solution: A 9 kW immersion heater and an AC•THOR 9s from my-PV represent the solution – with total costs of approx. 1,200 € which are a tenth of the heat pump!

For the installation of the AC•THOR 9s, the electrical connection through a separate circuit to the distribution box was necessary.

The 9 kW immersion heater could be integrated into an existing buffer storage tank, as the storage tank was prepared for an immersion heater. The conversion was correspondingly inexpensive to implement.

Why my-PV products?

For Mr. Nißle, it was crucial that the future solution could deal with excess electricity from the photovoltaic system and that it could be integrated into the existing energy management system. Because the energy flows are controlled with a Raspberry Pi on which openWB's EMS is installed.

An interface was programmed by a lively community, which can integrate and control the AC•THOR 9s via the TCP-Modbus interface.

Previous yield data and operational experience

The solutions from my-PV went into service with Mr. Nißle in time for the heating period in October 2021 and since then have been supplying the entire house with hot water and space heating. Based on the experiences of the first four and a half months, an annual consumption of around 3,200 kWh can already be estimated. In terms of energy, this would be almost half the consumption of the gas boiler.

Due to its functional principle, the immersion heater has converted almost the full electrical output into heat, and it is also fully linearly controlled by my-PV. The gas boiler (no condensing technology), on the other hand, had a significantly lower level of efficiency, since a considerable part is lost to the environment in the form of waste heat through the chimney.

With these consumption values, the my-PV system can be operated more economically than the existing gas boiler, even when it is completely connected to the grid (i.e. without a photovoltaic system). The following costs are also eliminated: gas connection or monthly basic fee, the costs for the chimney sweep, maintenance intervals for the gas boiler and the repair of the gas boiler.

Since the energy requirement for hot water and space heating is not insignificant in the winter months, the use of BYD's battery storage makes no sense for the customer during this time. For this reason, the following priority has been set for the surplus from the photovoltaic system:

First, connected electric vehicles (2 electric cars in the household), then the heating system from my-PV and only then the battery storage.

With this setting, the my-PV system replaces the BYD battery storage as energy storage in winter operation! Since the storage capacity of the buffer storage is very large, the battery storage is no longer used and after a few days it goes into an idle state. The customer has set the SOC to a battery-saving value of 50%. Advantages of the constellation: During the heating period, the battery storage is protected and the number of cycles per year is reduced, which means that a longer service life can be achieved. During this time, the buffer store is used as an energy store; it has a significantly larger storage capacity than the battery storage.

A little more detail for interested technicians

An even more detailed explanation of the system: The buffer tank has a tank-in-tank system. This means that the drinking water tank (160 l) is integrated in the upper area of ​​the buffer tank. The immersion heater is approximately in the middle of the buffer storage tank and thus directly below the drinking water bubble. The heating circuit for the central heating goes via the return line to the buffer tank and then back to the flow via the gas boiler. For the first year of operation with the immersion heater, the gas boiler remains as a backup; only the built-in circulation pump is used for the heating circuit.

The following settings were selected for the AC•THOR 9s:

• Operating mode heating mode M1

• Hot water backup: 50° Celsius, 8:00 a.m. to 10:00 a.m. and 4:00 p.m. to 9:00 p.m., 6 kW.

• Thus, in the period from 10:00 a.m. to 4:00 p.m. it can be fed by excess PV. In the hot water backup period, 3 kW remain for PV surplus

• Legionella program: 65° Celsius every 14 days, 9 kW

The network purchase is divided as follows:

• In the morning, 100% of the electricity is drawn from the grid to ensure hot water.

• When it comes to securing hot water in the evening, how much has to be drawn from the grid depends on the solar yield. On a sunny winter day, the upper area of ​​the buffer tank reaches up to 75 ° – so no mains electricity is required in the evening.

First degrees of coverage

The solar yield share was approx. 15% in the months of October 2021 to mid-February 2022. It should be noted that the weather this winter in northern Germany was exceptionally bad with little sunshine. According to Mr. Nißle, however, up to 20% of the solar share in the heating period would be possible with this system design.

Here are some statistical evaluations:

Outside temperature  Energy requirement for hot water and heating per day

• -10° bis 0°: 30–35 kWh

• 0° bis 5°: 25–30 kWh

• 0° bis 5°: 25–30 kWh

• 5° bis 10°: 20–25 kWh

• 10° bis 15°: 10–20 kWh

• über 15°: unter 10 kWh

Monthly consumption values for hot water and heating

• Oktober 2021: 172 kWh        

• November 2021: 534 kWh

• Dezember 2021: 836 kWh

• Januar 2022: 800 kWh

• Februar 2022: 360 kWh (bis 16.02.22)