68% solar fraction in the annual energy balance of the Singen Microhouse

Research project provides reliable measurement results

The research project "IR-Bau 2 - Supplementary investigations into the potential of infrared heating systems" by Konstanz University of Applied Sciences (HTWG Konstanz) used measurement equipment to study a microhouse in Singen over two heating seasons. The project was headed by Prof. Dr.-Ing. Thomas Stark, Dr.-Ing. Jan Heider, Nicole Conrad and Niklas Bachmann; it ran from 11/2020 to 04/2023.

The measurements confirm my-PV's established design approach: In suitable buildings with a sufficiently sized PV system, continuously controlled direct heating can be combined with a controlled domestic hot-water tank in a technically sound and economical solution.

The 50 m² tiny house

The "Mikrohaus" residential building in Singen was designed in 2020 by Kupprion nachhaltige Architektur and built on a small infill site to increase urban density. The single-storey cuboid has around 50 m² of floor area and three rooms, providing space for up to two occupants; it is currently occupied by one tenant. The timber-frame building is insulated with wood fibre and features a solid timber ceiling that remains visible inside.

Inside, the microhouse is divided into three zones: a central living area and private rooms with a bedroom and bathroom arranged at the sides. The south facade is extensively glazed, while the other external wall surfaces are almost entirely closed.

A heat pump was initially considered for the heat supply. However, for this small building and its expected low heat demand, the investment cost of a heat-pump system was comparatively high. Instead, the building uses a direct electric heating system optimized for self-consumption.

Four components of the solar-electric heat supply

The system consists of four components:

• A 4.3 kW DEVI / Danfoss electric underfloor heating system with two heating circuits, embedded directly in the building's 22 cm concrete floor slab - in other words, thermal activation of the building structure.

• A 120 l electric flat water heater from Austria Email.

• An 8.6 kWp PV system with an east-west orientation and a 5° tilt, partly shaded by trees on the neighbouring property.

• The my-PV AC•THOR 9s connects loads and generation and maximizes self-consumption of solar power.

Two monitored heating seasons with conclusive results

The microhouse was instrumented and monitored in detail over two heating seasons. The results confirm that, in suitable buildings with a sufficiently sized PV system, continuously controlled solar-electric direct heating can be combined with a continuously controlled hot-water tank in a technically sound and economical system - while achieving high solar fractions.

Specific space-heating energy consumption in the year under review was 37 kWh/m²a, equivalent to an annual consumption of 1,834 kWh. The building has a very high insulation standard. Nevertheless, the surface-area-to-volume ratio, which is unfavourable for small buildings, and a ventilation system without heat recovery noticeably increase energy demand.

Hot water: storage losses as part of load shifting

Electricity consumption for domestic hot-water production was 694 kWh per year, or around 14 kWh/m²a. This is comparatively high for a single-person household. The main reason is the storage losses of the 120 l tank. At 1.48 kWh per 24 hours, they add up to as much as 537 kWh per year - around 77% of the electricity used for hot water.

These losses are a direct consequence of the selected control strategy: When surplus PV power is available, the building (concrete floor slab) and domestic hot-water tank are deliberately heated above their normal target temperatures. This creates thermal storage capacity and shifts electricity consumption to periods of high solar generation. The storage losses are therefore part of the intended load shifting.

Heating season dominates the annual energy balance

Around 91% of space-heating demand occurs during the heating season from October to April. December and January have a particularly strong impact, together accounting for around 41% of annual space-heating demand. These two months therefore largely determine the annual energy balance - both for consumption and for the solar fraction.

Threefold energy self-sufficiency on an annual balance

An 8.6 kWp photovoltaic system is installed. Despite partial shading and unusually low generation in December - caused by prolonged snow cover and fog around Lake Constance - annual generation reached 8,118 kWh. This corresponds to a specific yield of 990 kWh/kWp.

Around 36% of generation occurs over the full heating season, while December and January account for only about 3%. On an annual balance, the PV system therefore generated more than three times the electricity required for the heat supply:

• PV generation: 8,118 kWh/a

• Electricity demand for space heating and hot water: 2,529 kWh/a

• Around 21% of the solar power was used directly to meet heat demand.

Solar fraction: annual average of 68%

The project's key performance indicator is the solar fraction:

• 68% solar fraction for total heat generation (annual balance)

• 61% solar fraction for space heating

During the shoulder seasons, heat demand can be covered almost entirely by solar power. Depending on the PV yield, this is where the control strategy delivers its full effect.

In December and January, naturally low solar availability becomes more pronounced. This inherent system limitation affects every form of solar-based heat supply.

Annual performance factor of 3.16 with photovoltaics

Evaluation of the solar fractions and grid imports results in a calculated annual performance factor of 3.16 for hot water and heating with photovoltaics.

Relative to the grid electricity used, more than three times as much heat is supplied. Grid electricity consumption is comparable to that of an air-to-water heat pump - but at a decidedly different level of investment.

Scientifically confirmed: PV heating is future-ready

The "IR-Bau 2" research project shows that photovoltaic heating can play a major role in the heat supply of suitable buildings. This form of heat supply is also already established in law: Germany's Building Energy Act (GEG) recognizes PV-controlled direct heating as a compliance option under Section 71d.

The available measurement data demonstrate that the technology works and can be implemented economically. Similar regulatory openness would also be desirable in other countries-

The full final report of the research project is available at:
https://www.htwg-konstanz.de/fileadmin/pub/ou/energie/Forschung/IR-Bau_2/Abschlussbericht_Forschungsprojekt_IR-Bau_2.pdf