OPE Journal

OPV, SMART CITIES & SUSTAINABILITY 10 No 37 | November 2021 | OPE journal The existing market and technologies for VIPV Loic Tous: “In the nineties, the Volkswagen group introduced a small PV roof as an option in several cars such as the Audi A8, VW Phaeton, Skoda Superb, and others. These PV roofs were very expensive (>€2000) and had a maximum power of only 30 Watt peak (Wp) which was mainly intended to avoid ignition problems related to 12V battery drain. In the last decade or so, integrated PV technology has rapidly evolved and is becoming sufficiently cost-effective for larger-scale integration in commercial vehicles.” Today, the Toyota Prius Prime, Nissan Leaf, Karma Fischer Revero, Hyundai Sonata, and Ioniq 5 are some of the first cars that offer PV-integrated panoramic sunroofs either standard or on their option list. Other automotive brands in the premium segment are following suit. Soon, they will offer similar features, most likely as optional “solar packages,” similar to driver-assistance, business, and other comfort packages. With power outputs in the orders of a few hundred Watt, these PV panoramic sunroofs are mainly intended to support the heating, ventilation, and air conditioning (HVAC) systems. As such, they can annually result in around two to three thousand kilometres of extra range depending on the location and driving profile. Tous adds: “Because of the inherent nature through which automotive manufacturers produce large panoramic sunroofs, integrating them with PV systems is the logical next step. In large panoramic sunroofs, a polymer encapsu- lant is laminated in between two glass plates to avoid shattering in case of an accident. Laminat- ing additional solar cells in between the glass and the encapsulant adds limited complexity and costs and is, therefore, being seriously considered by the automotive industry.” But there is more to it than just gluing solar cells onto one of the existing layers of the sunroof. As a first example, one could look at the already mentioned lamination process. In automotive industries, this lamination is done in autoclave machines that operate at high pressures, which can destroy the PV components. In addition, the autoclave needs a relatively long process time to provide satisfactory results. In the PV industry, lamination is performed with simple membrane laminators, which operate at much lower pressures and are more optimised for higher throughputs and in-line processing. Conventional laminators, however, also have their drawbacks when aiming to apply them in automotive. Two-membrane laminator Loic Tous: “PV laminators typically use a membrane to apply pressure on one side of a PV panel that is supported by a flat carrier. If you put a curved glass panel inside without any support, the one-sided pressure causes it to break. We, therefore, have a state-of-the-art two-membrane laminator to our disposition at imec in which homogeneous pressure is applied simultaneously on both sides. Within the imec.icon SUNDRIVE project, one of the activities is to compare this approach with the autoclave processes used in automotive. Both approaches can deliver perfectly func- tional devices, yet the membrane lamination approach is most promising to move towards higher-volume manufacturing.” Similar examples and research efforts are found in the material choices for the encap- sulant (the layer that protects the PV cells from environmental hazards such as humidity, mechanical stress, etc.). The PV industry mainly uses ethylene-vinyl acetate (EVA) or polyolefin (PO) materials as an encapsulation layer. These materials are optimised for minimal penetra- tion of agents such as water or salts but less for impact resistance or acoustic properties. In automotive, the commonly used layer in between the two glass substrates is composed of automotive polyvinyl butyral (PVB), which is well optimised in terms of impact, transpar- ency, and acoustic properties (properties that are becoming crucial in “silent” electric cars). On the downside, it is more hygroscopic, so less suited to protect PV cells against humidity and salts. Tous explains: “Overall, one could say that there are two approaches. The industry-conservative approach starts from the existing automotive technologies, materials, and processes to make them compatible with the requirements for PV integration. That’s why for the moment, for PV sunroofs, autoclaves and automotive-PVB materials are being used and optimised for compatibility with PV technology. In the long term, the more scalable solution in terms of cost and throughput could be to start from the technologies, materials, and processes optimised for PV and tune these towards the automotive manufacturing chain. In the more mature – yet equally conservative - market of building-integrated PV (BIPV), we are already in the stage where construction companies have gained enough confidence to move away from architectural PVB and allow for the adoption of EVA or POE. I expect the same to happen in the automotive sector at some point.” The next step: co-design of car and PV system The big challenge, however, is to design and manufacture cars where PV integration can practically eliminate the need for regular charging. For the moment, this is the domain of startups and scale-ups such as Lightyear in the Netherlands and Sonomotors in Germany. With their respective cars, Lightyear One and Sion, they aim at generating sufficient energy from the integrated PV panels for the vehicle to be fully self-sufficient on short distances like a daily commute to work. Tous emphasises: “These companies are aiming at annual range extensions of ten thousand kilometres or more, enabled by the integrated PV modules. Compared to PV-enabled panoramic sunroofs, this would mean power outputs of over a thousand Watts instead of around two hundred Watts. Toyota has built prototype cars with high-efficiency PV cells that are optimised for space applications, but these are far too expensive to integrate into commercial vehicles. To reach these power outputs at an affordable price tag, one must integrate PV modules not only in the roof but also in other body panels. And also, related factors such as weight, aerodynamics, and friction of the tires become increasingly important. In contrast to the more straightforward and compatible process of PV integrated sunroofs, building a fully PV-powered car requires a new approach from the ground up whereby car and PV sys - tem have to be co-designed.” Developing the technologies to support this ambition is quite demanding. For example, in contrast to glass sunroofs, body panels come in a larger variety of materials (metal, plastics, fibre composites). Also, potentially integrated PV systems will need to pass all automotive tests, demonstrate reliability and Fig. 2: When aiming for PV-powered cars, you have to co-design the vehicle and the PV system and apply PV not only on the roof but also in the body panels. (Image by Lightyear)