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Photovoltaic Heat vs. Solar Thermal – Cost and Area Comparison

A comparison dating back to 2015 illustrates the evolution of both technologies and which one offers advantages in terms of costs as well as area.

"Why generate electricity first and then use it for heat? Isn't that the purpose of solar thermal?" Dr. Gerhard Rimpler frequently encountered this question and its reaction almost a decade ago. With the unconventional idea of using photovoltaic systems to produce hot water, his company my-PV initiated not less than a paradigm shift in solar heat generation. Since then, the motto "Cables instead of pipes" has revolutionized the solar market, with the product ELWA launched in 2014, marking the "Revolution in Solar Thermal"!

With a ninety percent lower copper consumption compared to thermal solar systems and significant price reduction in recent years, photovoltaic heat has long surpassed solar thermal both technically and economically. These are just two of the many advantages of photovoltaics. Here's an initial overview.

Price Differences Between Solar Thermal and Photovoltaics

Since 2015, we have been conducting price comparisons for heat generated through photovoltaics versus solar thermal systems. For this purpose, we always compare a current photovoltaic module with a commercially available solar thermal flat collector.

Based on the graphic, it is evident how the price development has evolved in recent years:

  • The price for the flat collector remained practically unchanged between 2015 and 2020. The reason for this is the use of raw materials such as copper and aluminum, which are still used in large quantities in production.

  • In contrast, there was a significant decrease in prices in photovoltaics until 2020, with a price reduction of around 65 percent per watt peak of the modules. Unfortunately, in 2022, this trend has reversed: Due to the difficult political and economic situation, prices for both solar collectors and photovoltaic modules have risen again – both technologies to almost the same extent. Additionally, there was an unfortunate increase in costs for craftsmen, if any were found with available capacities at all. Installation costs (apart from the craftsman) and maintenance effort when using photovoltaics remain – compared to solar thermal – however, vanishingly low. Pipe lines, pumps, valves, expansion vessels, and antifreeze are eliminated.

  • In 2023, the prices for photovoltaics fortunately resumed their original trend – and faster than before 2022, while the price for solar thermal is still in an upward trend. The costs per watt peak of photovoltaics have thus returned to pre-war levels. From 2015 onwards, this marks a cost reduction of 65 percent again – triggered by price declines and performance improvements!

Resource Conservation in Solar Thermal vs. Photovoltaics

Compared to solar thermal systems, photovoltaics offer significant resource-saving potential for hot water preparation. Just in terms of the piping required for energy transmission from the roof to the hot water storage, photovoltaic heat provides savings of over 90 percent in copper consumption.

Differences in Operation

Both photovoltaics and solar thermal energy harness energy from sunlight. However, there is a clear distinction: Photovoltaic systems generate electricity, while solar thermal systems produce heat. In photovoltaics, solar cells, grouped into modules, are used for electricity generation. Solar thermal, on the other hand, utilizes collectors for heat generation.

Good to know

An example calculation: Let’s take a standard 2.25 mm thin electrical cable (diameter without insulation) and compare it with a copper pipe with a 16 mm inner diameter. The cross-sectional area of the cable is 4 mm², while that of the pipe is 53 mm². The copper consumption for the piping in photovoltaic heat is thus 93% lower compared to solar thermal systems.

The question here is: Do we need electricity or heat – or both? The direct current generated by photovoltaics is converted into alternating current by an inverter. This can be used in various ways in households – including heat generation! Photovoltaic electricity is either consumed immediately, directed to a battery storage, fed into the power grid, or indeed used for heat processing.

This diverse use of heat from solar thermal is not possible. Energy from solar collectors can only be used for heating purposes.

Differences in Efficiency – and the Associated Misconception:

Solar collectors theoretically convert a maximum of 80 percent of solar energy into heat. However, some of this energy is lost during the subsequent circulation of heated water. Therefore, solar thermal systems are assumed to have an effective system efficiency of about 50 percent. Crystalline photovoltaic modules, on the other hand, convert approximately 20 percent of solar energy into electricity, with minimal losses. Hence, the 20% figure is commonly referred to as the overall efficiency of photovoltaic systems.

However, this purely numerical comparison of efficiencies is not truly relevant. Firstly, the energy source is inherently free, and secondly, rooftops typically offer ample space to easily compensate for the difference!

What about the direct area comparison?

However, at my-PV, we also do not shy away from direct area comparisons. How the ratio of required photovoltaic module area to solar thermal collector area has developed over the years at the same hot water coverage level has been analyzed by my-PV, using a dynamic comparison simulation with the Polysun software. Our direct current solution, ELWA, an autonomous heating rod for heat from photovoltaic electricity, is compared to a solar thermal flat collector system with six square meters. Both technologies channel solar energy into a 400-liter hot water tank, with hot water extraction and reheating being identical for objective comparability in the simulation program.

To illustrate the trend of continuously improving module efficiencies, contemporary module types were set in the simulation for each observation year. While in 2015, the standard was still at a module output of 250 Wp on an area of 1.65 m², it is already at 400 Wp on an area of 1.92 m² in 2023. This corresponds to approximately 200 Wp per square meter in 2023, while in 2015, it was only 150 Wp per square meter.

In 2015, eight modules totaling 13.2 m² were required, whereas in 2023, only five modules covering 9.6 m² are needed to achieve the same level of solar hot water coverage as a solar thermal system with a 6 m² flat collector. The factor of space requirement is the only remaining technical advantage that solar thermal energy still has over photovoltaic heat today. However, even eight years ago, this argument was no longer of practical relevance because homeowners typically have more than enough roof space available to install a PV heat system.

Beyond this, the list of technical advantages of solar electric heat generation over traditional pipe-bound technology is long. A brief overview is always available on the my-PV website.

In addition to the remarkable learning curve regarding costs and space factor, the abundance of technical advantages of photovoltaic heat generation is well known. Elaborate pipe systems are replaced by thin cables. In comparison to solar thermal energy, this enables a much simpler and nearly lossless energy transfer.

Good to know

For us at my-PV, it is clear: Photovoltaics are more versatile, cost-effective, and environmentally friendly.

Dr. Gerhard Rimpler, CEO of my-PV, states: "Through my many years of work in the field of solar thermal energy, I became aware of the limits and drawbacks of this technology early on. To this day, there has been no significant reduction in costs. In contrast, photovoltaics have become an affordable product for the mass market for years."

References

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