Solar thermal energy in existing buildings

Key to the energy turnaround

Author: Dr. Roger Corradini / Forschungsstelle für Energiewirtschaft e.V. ; www.ffe.de; Date: April 2017 The energy transition in Germany focuses largely on the electricity sector. Yet only 25 percent of energy consumption is caused by electricity applications. More than 50 percent is attributable to heat applications. In addition to necessary successes in the transport sector, significantly higher shares of renewable generation must also be achieved in the dominant heat sector.

The energy turnaround in Germany is currently limited exclusively to an electricity turnaround, although only a quarter of final energy consumption in Germany is caused by electricity applications. In addition, the transport sector and, with just over 50 percent, the clearly dominant heating sector are of a similar magnitude. The annually increasing share of renewables in electricity generation to over 25 percent in 2013 attests to the visible success of the electricity turnaround. In contrast, the renewable shares for transport (6 %) and heat (9 %) have stagnated for years at a significantly lower level. In addition to the necessary successes in the transport sector, significantly higher shares of renewable generation must be achieved, especially in the dominant heating sector. Only if this goal is met, the project of the energy transition is ultimately holistically led to success.

Solar thermal energy: a key to the turnaround?

Can solar thermal energy make significant contributions to the urgently needed heat turnaround? This question was answered in a multi-year research study [1], in which the solar thermal potentials for existing buildings with one residential unit were determined. With new construction and modernization rates of significantly less than 1 percent per year, it is essential to substitute fossil fuels in existing buildings at a much faster rate. The results of the work show a solar thermal substitution potential of 78 TWh for a scenario with 20 m² collector area per system, even without any modernization measures on the building itself. This corresponds to around 25 percent of the heat consumption of this class of building, or an avoidance of up to 20 million tons of CO2. With simultaneous insulation or window replacement - cases that were not explicitly examined in the study - the achievable savings could be increased even further. Thermal long-term storage or plants with more than 20 m² collector surface could increase these values likewise still.

Accelerating the heat turnaround

On buildings with one residential unit, solar thermal energy is in direct competition with photovoltaics. The average area required by a PV system compared to a solar thermal system (see Fig. 2) generally leaves no room for a solar thermal system. In the context of the energy demand for heat being more than twice as large as for electrical applications, it is incomprehensible that for years there has been a clear subsidy policy preference for solar electricity generation over solar heat generation, which still exists today. Despite the considerable reduction of the feed-in tariff over the last years to currently approx. 12.5 ct/kWhel (Jan 2015), the subsidy in the thermal sector is only 2 ct/kWhth and thus less than 1/6. Since this ratio was significantly larger in the past, it is understandable that the annual construction figures of solar thermal are significantly behind those of PV, although the heat transition as an essential part of the energy transition would urgently require the implementation of these potentials.

Investment support such as the market incentive program for solar thermal energy, which is based exclusively on collector size, does not really do justice to the desired heat turnaround (cf. [2]). The goal is to avoid as much fossil energy use as possible through efficiency measures or appropriate regenerative technologies such as solar thermal. Here, it would be obvious to provide this avoided amount of energy directly with a remuneration. For the Solarthermie this could mean that the usable solar yield of the plant would be recompensed and thus indirectly the avoided gas or oil consumption.

As positive side effect well planned and maintained or with innovative regulations provided - thus generally efficiently functioning plants - a higher remuneration would receive than such, which have large collector surfaces, but for example by a bad hydraulic integration or regulation only few yield supply. There would therefore be a significantly higher incentive for quality-assured system operation. The level of solar thermal subsidies, currently the equivalent of 2 ct/kWhth of usable heat, also needs to be reconsidered in the context of an accelerated heat turnaround.

Solar thermal energy: an important and necessary key to the turnaround!

The energy turnaround in Germany is currently subject to a strong political and media focus on the electricity sector. With superficial view it reduces itself even completely to the current range. However, only 25 percent of energy consumption in Germany is caused by electricity applications. More than 50 percent is attributable to heat applications.

Both socially and politically, it is important to communicate this fact and to demonstrate resilient solutions through appropriate scientific work - and also to address selected aberrations. For example, the increasing use of heat pumps to heat buildings is only a silver bullet at first glance. At times when heat pumps have the greatest demand for electricity - January and February - the grid load is among the highest, and renewable power plants certainly do not supply their largest shares of the electricity mix. In the future, a very large increase in the number of electric heat generators would result in excessive power peaks during the cold months of the year. For this, it is necessary to provide secured power for renewable generators such as wind and photovoltaics, which are not always available. This can be done, for example, using large-scale electricity storage or reserve power plants. However, the cost of this additional energy infrastructure will have to be borne by each electricity customer, similar to the EEG.

Solar thermal, on the other hand, has no influence on the oil or gas price that the neighbor has to pay. Thus, solar thermal energy can not only be certified to be an important building block of the heat turnaround, but moreover, it is a necessary key to limit the overall societal costs of the energy turnaround.