The purely solar-electric house – its environmental impact compared to the heat pump

Motivation

Austria has committed to reducing its emissions by 36 % by 2030 compared to 2005. Currently, many climate activists doubt the achievement of the set climate targets. With increasingly radical forms of protest - such as sticking on roadways and pouring actions in museums - they want to attract attention and call on politicians to act. The scientific community, especially Scientists for Future, is campaigning for swift action on climate change and demanding concrete measures. Global warming and CO2 emissions have also become an omnipresent topic in newspapers and online news portals, not least because of natural disasters or measurement records.

Buildings accounted for 10.56% of total greenhouse gas emissions in 2022, with the majority coming from the burning of fossil fuels. Apart from environmental concerns, recurring energy crises have also increased interest in building technology among the population, and photovoltaic systems have experienced a real boom since 2022. More and more people also want to "get out of oil and gas" and are turning to pellets and heat pumps. However, it must be taken into account that most households only have a limited budget for retrofitting and this influences the choice of possible measures.

The scientific work therefore addressed the question of how effective different building energy systems are in terms of ecological impact.

Purpose and methodology of the work

The aim of the work was to evaluate two different building energy systems using the example of a fictitious single-family house with regard to their ecological impact. The systems in question are as follows:

- Execution 1: A direct electric underfloor heating and hot water system in combination with a photovoltaic system.

- Execution 2: A water-guided underfloor heating and hot water heating system that draws energy from an air-to-water heat pump. In this case, the electricity is drawn entirely from the grid.

In order to determine the environmental impact of products or materials, life cycle assessment (LCA), also known as life cycle analysis, was used. In this work, a simplified form, an LCA screening analysis, was carried out. The environmental impacts of production and operation were determined and mapped; disposal was not taken into account. The comparison looks at the global warming potential (GWP). This describes the quantities of CO2 and other greenhouse gases emitted.

The period under consideration is 30 years, and the fictitious single-family house is located in Linz, Austria. The single-family house has a living space of 120 m² (gross floor area 150 m²), a heating demand of 25 kWh/(m²a) related to the gross floor area (GFA) and year. The assumed hot water demand for 4 people is 200 l per day. In order to be able to compare the overall system, the household electricity of 4000 kWh per year was also taken into account. A service life of 15 years was assumed for the heat pump, i.e. replacement over 30 years.

Literature research was used to collect data on the individual components of the systems and a comparison of different variants was carried out on this basis.

Conclusions of the literature research

It was found that photovoltaic modules account for by far the largest share of emissions from design 1 (direct electric underfloor heating and domestic hot water production with photovoltaics), accounting for about 70-75% of the GWP.

Comparing the GWP of PV modules from China with production under European electricity generation conditions, it can be calculated that about 300 kg CO2 equivalent per kWp could be saved with more ecological electricity generation. In this comparison, the Chinese electricity generation mix was valued at 836 g CO2-eq. per kWh. For European production, the electricity mix for silicon production from Norway (with 30 g CO2-eq per kWh) was used, and the European average of 418 g CO2-eq per kWh was assumed for the remaining process steps.

The global warming potential for the production of refrigerants for heat pumps is relatively low (for example, the value for the most commonly used agent in 2021, R-410A, is 10.7 kg CO2 equivalent per kg). However, the refrigerant R410a has a global warming potential of 2088 CO2 equivalent per kg when released into the atmosphere. Literature data on leakage varies between 2 % and 8 % per year, in this work an annual leakage rate of 3 % was assumed.

Emissions from electricity generation vary greatly in individual countries. In Austria, the emission factors for the electricity mix are 227 g CO2-eq./kWh, while in Germany a value of about 400 g CO2-eq./kWh has to be considered.

In the course of the scientific work, the German electricity mix was not yet taken into account. These calculations were carried out subsequently.

Results of the comparative variants

In the course of the scientific work, several factors were varied, such as the output of the photovoltaic system, the heating demand (HWB) of the building, the refrigerant used in the heat pump or also future changes in the composition of the electricity mix. These variations lead to the illustration of 24 different variants.

In a variant with a 10 kWp photovoltaic system on the building with the electric underfloor heating (version 1) and a building with a heat pump without a photovoltaic system (version 2) with the refrigerant R-410A, as well as the Austrian electricity mix, version 1 with the electric underfloor heating and the PV system performs better than version 2 with the heat pump. The higher emissions for the production of the PV modules are compensated for in Austria after 14 years, in Germany already after 4 years.

How can it be that a purely solar-electric house (version 1) in Germany, despite higher CO2 pollution of the grid electricity, performs better ecologically than at a location in Austria?

Due to the crediting of the feed-in of photovoltaic power surpluses in the CO2 balance. In the scientific work, the approach chosen was to value the surpluses fed into the grid as a credit with the respective monthly conversion factor of the electricity mix, a factor that defines the global warming potential (GWP) per kilowatt hour over the course of the year. The conversion factor in Austria in the summer months is only one third of the conversion factor in winter; in Germany this discrepancy is not as pronounced. Despite these differences, the result is that a high summer grid feed-in more than compensates for the disadvantage of lower electricity purchases in winter.

Direct electric heating systems in combination with a photovoltaic system in low-energy buildings have a lower environmental impact than water-based heating systems with heat pumps!

The graphs show that the initial production costs for the photovoltaic system account for a large share of the global warming potential; during further operation, the environmental impacts are comparatively lower. With the heat pump, on the other hand, the share of production is very low, but not so during use. Here, the greenhouse potential of the purchased electricity is the decisive factor.

For comparison: with a heating requirement of only 15 kWh/(m2a), above which buildings are also called passive houses, the intersection point is reached after only 10 years in Austria, and again after 4 years in Germany.

The systems with a 10 kWp system are of course even better in terms of global warming potential for buildings with a HWB of 15 kWh/(m²a).

In this case, the better the house, the weightier the share of emissions produced in the manufacture of the PV modules and the higher the share of emissions due to leakage from the heat pump.

If the specific heating demand of the house is raised to 80 kWh/(m2a) in the calculation and the current Austrian electricity mix is assumed, heating systems with heat pumps are superior to electric systems. This also applies to electric systems with a 15 kWp PV system. The intersection is no longer reached, even with German electricity mix.

The primary heating system should only be designed to be purely solar-electric in residential buildings that have been built or thermally renovated according to the current state of the art!

The CO2 equivalent emissions of the electric underfloor heating with a 10 kWp PV system were found to be very strongly dependent on the heating demand. This difference is not as pronounced for the variants with a heat pump.

Conclusion of the scientific paper

The study showed that heating systems with electric underfloor heating in combination with contemporary, roof-filling photovoltaic systems produce lower greenhouse gas emissions in buildings with a low heating demand than heating systems with a heat pump (refrigerant R410a). When small photovoltaic systems are installed, this picture is inverted and there is an advantage in favour of heating with a heat pump. Only in passive houses are small photovoltaic systems sufficient to achieve lower greenhouse gas emissions with purely electrical systems than with systems with a heat pump.

On the other hand, for buildings with higher heating requirements, heating systems with heat pumps have an advantage under Austrian electricity conditions. In other European countries with an ecologically inferior electricity mix, in individual cases, if the photovoltaic system can be dimensioned large enough, the electric heating system could cause lower greenhouse gas emissions due to the summer grid feed-in to be taken into account.

It was also shown that refrigerants with a high greenhouse gas emission potential have a large influence on the overall emissions of a heating system. Especially in buildings with a good insulation standard, the share increases; but also with a higher heating demand, which requires a larger heat pump with a higher charge, the share is not negligible. From this it can be concluded that the conversion of heat pumps to refrigerants with low global warming potential is urgently needed for the success of the climate targets.

In conclusion, it can be stated that the installation of large photovoltaic systems in combination with electric heating systems is better than a system with a heat pump without a photovoltaic system under the ecological aspects considered here for single-family houses with low heating requirements.

The complete bachelor thesis is also available for download.

Annotation from my-PV: Why not consider combining a heat pump and photovoltaics?

There are several reasons for this: On the one hand, the electrical drive power of heat pumps is often not infinitely variable, but this is a fundamental prerequisite for the use of surplus electricity from photovoltaics. With purely electric heat sources in combination with suitable power controllers, on the other hand, heat generation is "PV-ready".

Another essential aspect is the cost. Two systems (PV and conventional heating combined) are of naturally correspondingly more expensive and often too great a financial hurdle for the broad mass of builders and homeowners in a time of high inflation. In addition, there are the monetary expenses for the maintenance of the heat pump. If the decision is made in favour of the heat pump, however, the photovoltaic system is often seen as a later add-on that can only be added years after construction.

Third point: noise. Building technology that does without moving parts is not only low-maintenance, but also low-noise.

The production location for the solar panels is essential for the ecology of purely solar-electric houses. It is therefore advisable to relocate the production sites for modules back to Europe.