**1. Introduction**

The reduction of harmful greenhouse gases is one of the biggest challenges of our time. The use of unlimited available solar thermal power as renewable energy can help to support efforts to reduce CO2 emissions worldwide. More than 40% of the total energy consumption in Europe can be allocated to heating, hot water, and illumination [1]. This corresponds to almost 20% of the total CO2 emissions. Especially in the building industry, reducing CO2 emissions is one of the main drivers. By using modern building envelopes, the required energy for heating and cooling can be minimized [2]. The major goal is the realization of affordable zero-energy buildings. One effective way of achieving these objectives is the use of solar thermal energy. The solar radiation at our latitudes is perfectly suited for thermal use.

In central Europe, solar radiation produces, ona1m2 area, 1000 kWh/year to 1100 kWh/year [3], which is equivalent to 100 L of fuel oil or 100 m<sup>3</sup> of natural gas. For this reason, the building sector has a strongly growing interest in facade-integrated solar thermal absorbers, which shall be presented in this paper.

A broad range of solar thermal collectors have been available on the market for decades worldwide. These products are credible and generally made to a high technical standard, especially in Europe with a homogeneous market. However, there is a shortage in the field of solar thermal products that are suited for building envelope integration to create high-quality architecture [4,5]. Functional and constructive aspects, together with aesthetics, have to be considered. State-of-the-art solar thermal collectors are not flexible in shape and size due to the hydraulic fluid circuits fixed to solar absorbers. Architectural requests for design freedom require the hydraulic system concept to be redesigned, which is generally difficult and expensive for conventional means of production. Conventional production methods for absorbers limit the flexibility in collector design [6] and such a lack

**Citation:** Scholz, P.; Weise, D.; Schmidt, L.; Dembski, M.; Stahr, A.; Dix, M.; Duminica, F.; Le Craz, S.; Koziorek, J. Sheet Metal Design Approach for 3D Shaped Facade Elements with Integrated Solar Thermal Functionality. *Solar* **2023**, *3*, 213–228. https://doi.org/10.3390/ solar3020014

Academic Editors: Luis Hernández-Callejo and Jürgen Heinz Werner

Received: 13 January 2023 Revised: 3 April 2023 Accepted: 6 April 2023 Published: 13 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of flexibility significantly reduces the potential application of solar thermal systems [7]. The production of solar collectors in sheet metal half-shell design by means of hydroforming was investigated in the European Union (EU) project with the acronym "BIONICOL" [8] and in "Industrielle Gemeinschaftsforschung" (IGF) project No. 339 ZN [9]. However, curved surfaces cannot be obtained with the shown technologies and the channel structures can only be manufactured on both sides, which has a disadvantageous effect on the design freedom of the facade. The integration of flat-plate solar thermal collectors is only possible for opaque envelopes (roofs and facades) so far [10]. The structure of evacuated tubes allows mounting on transparent envelopes as sun shading, but this kind of application is rare. The application potential of incremental sheet forming (ISF) in solar absorbers was demonstrated in [11] by the production of absorber lamellae using incremental forming technology.

The thickness of solar collectors affects their integration, especially for facades. Thick solar thermal collectors are difficult to implement as functional elements or as sun shading. The appearance of solar collectors is affected by the glass (glazed collector) and absorber surface treatment. State-of-art collectors apply highly transparent, low-iron glass for glazing and black or dark-blue spectrally selective coatings with high absorptance (0.95) and low emittance (0.05). Solar collectors integrated into facades are more conspicuous than collectors installed on rooftops. Many studies and surveys have shown that architects prefer a large variety of absorber colors [12], and they even regard the possibility of a custom color choice as essential. Manufacturers can meet the demand for a variety of absorber colors by means of solar paint coatings, but such collectors show considerably reduced thermal performance compared with quality selective coatings in the usual solar collectors commercially available in Europe [13].

This paper presents a new design of a solar thermal collector. The focus of this paper is fabrication with ISF, concepts for integration into facade systems, possible design arrangements, and the development of the absorber coating. Furthermore, initial results on the thermal efficiency and performance of the new collector type are presented and evaluated.

#### **2. Materials and Methods**

#### *2.1. Principal Collector Design*

Figure 1 shows the layout of the developed collector design, which consists of two layers of cold-rolled deep-drawing steel DC06 (t = 0.8 mm) joined together in a fluid-tight manner by laser welding.

The three-dimensional shaped outer sheet is designed as a closed cassette with integrated notches for hooking into the façade's substructure. To integrate the solar thermal functionality, the flat inner sheet has a two-dimensional channel structure and is attached to the outer sheet. The use of this simple geometry for the inner sheet ensures a good fit between the two components, which is essential for the joining process. The channel structure chosen was a harp structure, which has also been used in commercially available flat plate collectors. With this structure, the area to be cooled can be efficiently passed through with low pressure loss at the same time. For hydraulic connection, parallel planar surfaces were proposed in the inlet and outlet areas for the easy integration of conventional fittings.

The representative design of the geometry of the exterior sheet was determined by the results of the solar gain analysis, the architectural design boundaries, and the constraints of the forming process, which are described in the following sections.
