District heating with photovoltaics
Utilizing PV surplus energy at the district heating plant of Stadtwerke Straubing
At the district heating plant in Germany, heat is supplied to a town hall, a nursing home and 100 residential units.
Facts about the project
- Owner
- Photovoltaic capacity
- Number and type of modules
- Thermal Storage Size
- Building Type
- Heat generators of the district heating plant
- Installed my-PV Product
- Heating Element
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At the district heating plant operated by Stadtwerke Straubing, the high fuel demand represented a key challenge: when outdoor temperatures were around the freezing point, approximately 130 m³ of wood chips per week had previously been required to maintain reliable heat supply. Against this backdrop, the goal emerged to further increase the share of renewable energy and consistently utilize available roof areas. The objective was to convert surplus solar energy directly into heat and thereby relieve the existing biomass plant—deliberately independent of the public electricity grid and without additional grid infrastructure.
Grid-independent PV direct heating with my-PV
The core of the implementation is a fully grid-independent DC solution for generating heat from solar power.
With the my-PV DC Power Manager SOL•THOR, direct current is converted into hot water (domestic hot water or heating water) whenever the sun is shining. In Straubing, a photovoltaic system with a capacity of 32.04 kWp was installed. A total of 72 modules of the type Trina Vertex S+ TSM-445NEG9RC.27, each with an output of 445 Wp, are used.
Personal customer opinion and resumee
Stadtwerke Straubing is very satisfied with the system:
“From the implementation to the ongoing operation, everything has run smoothly and without any problems. Mr. Holzhauser, the contact person from the executing company GH-Holzhauser Electronic GmbH, has always been very committed,” says Stefan Manger (Project Engineer for Heat Supply at Stadtwerke Straubing Strom und Gas GmbH).
The autonomous PV system is divided into nine smaller subsystems, each equipped with one SOL•THOR unit from my-PV. This configuration enables the solar yield to be continuously and smoothly converted into heat for the municipal utility’s heat generation.
Each SOL•THOR is connected to two strings of four modules each and controls a 3 kW heating element, the my-PV screw-in heating element 3 kW. In total, nine heating elements with a combined output of 27 kW are used.
A special feature of the installation is its consistent grid-independent design: the generated solar electricity is converted directly into heat without an inverter. There is no feed-in to the public grid and no grid consumption for this part of the system.
This direct use of DC electricity eliminates conversion losses from AC conversion, significantly increasing system efficiency while also simplifying the technology.
Integration into existing heating system
The generated heat is fed into three existing buffer storage tanks, each with a capacity of 20,000 liters. Each storage tank is equipped with three heating elements and the corresponding SOL•THOR power controllers.
Mechanical integration was achieved using a specially manufactured flange plate, which was installed in the existing access opening of the storage tanks. This allowed the solution to be implemented without extensive modifications or structural interventions in the existing storage system.
The generated heat is stored directly in the heating water and then supplied to the existing district heating network.
The PV direct heating system operates in parallel with the biomass plant: when solar radiation is sufficient, it takes over part of the heat generation and directly reduces wood chip consumption. Particularly during transitional seasons and sunny winter days, the system noticeably relieves the main heat generators.
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System data of Stadtwerke Straubing
At the district heating plant of Stadtwerke Straubing, heat is supplied to numerous public and private consumers. The district heating network operates with a supply temperature of 70 °C and supplies, among others the town hall, a retirement home, several other public buildings and around 100 residential units
In addition to the 27 kW DC heating solution from my-PV, the following heat generators are used:
2 combined heat and power plants (CHP) with 90 kW each
1 heat pump with 900 kW
2 gas boilers with a combined output of 1700 kW
1 biomass boiler with 900 kW
Additional grid-connected PV system
In addition to the grid-independent direct heating solution, another photovoltaic system with 29.37 kWp has been installed at the Straubing district heating plant. This system is grid-connected and serves to optimize on-site electricity consumption in operation. The system size was deliberately kept below the 30 kWp threshold.
To increase self-consumption, the system is equipped with battery storage from LG Energy Solution. This installation operates completely independently of the my-PV solution for heat generation from photovoltaic electricity and is used exclusively for electricity supply and load optimization.
In this case, the typical my-PV approach of increasing self-consumption through PV heat is not applied. Instead, a separate autonomous DC solution with its own PV system was chosen.
Result: More renewable energy, less fuel
With the combination of grid-independent PV direct heat generation and a supplementary grid-connected self-consumption PV system, a holistic energy concept has been implemented.
The direct conversion of solar electricity into heat measurably reduces wood chip consumption, lowers operating costs, and increases the share of renewable energy in the overall system. At the same time, the existing infrastructure remains intact and is meaningfully complemented.
Initial yield measurements as of the end of March show that since August 2025, 9,000 kWh of heat have already been generated from solar electricity using direct current. This output is expected to increase further, as the high photovoltaic yields of spring are now beginning.
Stadtwerke Straubing is very satisfied with the system:
“From the implementation to the ongoing operation, everything has run smoothly and without any problems. Mr. Holzhauser, the contact person from the executing company GH-Holzhauser Electronic GmbH, has always been very committed,” says Stefan Manger (Project Engineer for Heat Supply at Stadtwerke Straubing Strom und Gas GmbH).
The project clearly demonstrates how large thermal storage systems and PV direct heating solutions can be efficiently combined—with minimal losses, robust technology, and independence from the power grid. It highlights the potential of photovoltaic heat generated directly from DC electricity as an economical and future-proof addition to existing heat generators in commercial and municipal applications.
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SOL•THOR
in use
The DC Power Manager converts solar power directly into heat – efficiently and with minimal loss by using direct current from PV modules to power a heating element.
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