Annual review of solar-powered apartment building

The purely solar-electric building technology already beats the heat pump by far in the annual energy balance. This applies to low-energy buildings with powerful photovoltaic systems and precise power modulation of the electrical heat generators. my-PV has already shown this in the past for single-family houses and commercial properties.

The project in Southern Styria

In a new apartment building with eight residential units, in each case one "AC•THOR 9s" control device from my-PV uses excess energy yields from a large, grid-connected photovoltaic system for decentralized water heating in the apartments and for electrical room heating. Exclusively the my-PV technology makes ordinary electric heat generators suitable for photovoltaics. The fast and precise power modulation makes them "PV-ready". Missing energy, for example in winter, is drawn from the public power grid. In times when the solar radiation is too low, the automatic temperature protection comes into effect – both for hot water and for room heating. For most of the year, however, this is rarely or not at all necessary. Other heat sources are also not needed.

The building technology is extremely simplified through the applied principle "cables instead of pipes". Technically, this my-PV guiding principle offers a long list of advantages, especially compared to traditional solar thermal systems. The most important advantage, however, is that clean solar energy is only converted into heat where it is needed. This avoids any heat losses in the pipes, and the installation effort for the cabling is comparatively low. With the decentralized electric hanging storage tanks in the apartments, thanks to the my-PV operating mode, there is always a storage reserve in addition to ensuring a minimum temperature. The hot water boiler thus becomes a "day storage" for photovoltaic surplus.

The background

The first contact between the developer and my-PV took place several years ago at the "Intersolar" trade fair in Munich. A few years later, when the product portfolio of my-PV included further areas of application, such as solar-electric space heating, the property developer got in touch with my-PV again.

During the first joint planning steps, the partner companies were initially sceptical about the new “photovoltaic heat” technology. For the planners and construction supervisors, the solar-electric concept was something they were not familiar with. However, in a joint meeting of all those involved in the project, the concerns were quickly dispelled. The ecological, technical and economic advantages of "cables instead of pipes" are very convincing for experienced construction companies. The tenets did not express any reservations regarding the concept and the authorities as well issued the permits without any difficulties, not even because of the electric heating!

Annual energy balance

In the 12 months between April 2021 and March 2022, an average of 2,068 kWh of electricity was consumed per apartment. This amount of energy includes the normal electrical household consumers such as kitchen appliances, entertainment electronics and washing machines, as well as the lighting, of course. 692 kWh of energy was supplied to the decentralized electric hanging tanks for hot water preparation. A value that initially appears very low in comparison, but is plausible for frugal users due to the avoidance of all heat distribution losses through circulation lines. 2,656 kWh were used for the electric space heating. This underlines the quality of the of the energy-efficient building standard.

Figure 1: Mean annual energy balance of eight apartments

More than half of the energy for these three sectors is self-generated thanks to the on-site photovoltaic systems! With a PV self-consumption of 2,814 kWh and a grid purchase of 2,601 kWh, this means a degree of self-sufficiency of 52%!

2,444 kWh could not be used on site and were fed back into the public power grid. This results in a PV self-consumption of 54%.

When considering purchased electricity and grid feed-in, it is noticeable that almost as much electricity was fed into the grid during the observation period, as was drawn from it. This means that the apartments are almost energy self-sufficient at 94%. However, my-PV does not just consider the electrical consumers, but as well always includes hot water and space heating in the evaluation.

Building data

• 8 residential units

• A total of 64 kWp, facing south, east and west, 10° inclination

• One electric hanging storage tank per unit, 150 litres, heating output 2.3 kW

• Area of the ​​apartments 54 – 70 m²

• Heating demand at location climate: 36 kWh/m²

The future of home technology

Experts recognized the revival of electric heating more than half a decade ago. As early as in June 2015, the magazine "Wirtschaftswoche" published a corresponding forecast by scientists from the Fraunhofer Institute:

"Power-to-heat is the future in the heating market" [...] If electricity is not increasingly converted into heat in the future, the energy transition might fail.

Just as large power plants decades ago produced superfluous electricity during the night, it is now the sun that supplies more energy than is necessary when the weather is right. In our houses, this now offers the possibility of making a "day storage" out of the hot water boiler and the component masses. The heat generators required for this have been around for a long time already, but now their linear power control is also highly developed and, thanks to the low prices, affordable for everyone.

Only with the precise, linear output control, the electric heat generator became a modern, photovoltaic-capable heat generator. This is what makes it "PV-ready"!

Water heating with solar power has only been the beginning. For modern buildings with up-to-date insulation standards and appropriately dimensioned PV systems, there is the option to abandon conventional, water-based heating systems from now on. This goes hand in hand with a significant "de-technicalization", because the jumble of pumps, valves, pipes and large-volume buffer storage tanks is becoming obsolete.

The builder's conclusion

For this new project near Graz in Austria, the developer used my-PV technology for the first time. So far, conventional hydraulic systems such as ground or air heat pumps, gas, oil or district heating have always been used in building services. In addition to the drastic simplification of the system technology, the areas of electricity, hot water and room heating can now be combined for the first time through sector coupling, because in the house of the future everything is purely electric, mostly solar-electric.

There are also no additional costs for tenants: "The living costs are either in the usual market segment – or even below it," says the developer.

The my-PV technology is easy to implement and is already planned for the next projects of the Styrian property developer. His conclusion after the first year of operation: "Once the concept has been understood by people after initial discussions, the question always arises: Why doesn't everyone do it?"