Nowadays, photovoltaic (PV) capacity in Poland is growing rapidly. While in 2010 the cumulated capacity was close to 0 MWp, currently it has exceeded 3.5 GWp [
1]. Moreover, almost 2.5 GWp was installed in the previous year. Subsidies from the public funds (the “Mój Prąd” programme [
2]) and increasing electricity prices encourage people and companies to invest in photovoltaic systems. Unfortunately, in the case of some large-area buildings, a problem arises due to a low roof load ability [
3,
4] which forecloses the mounting of conventional PV modules. Usually, this issue touches new modern warehouses and production facilities covered with membrane roofs which sometimes have a load capacity lower than 10 kg/m
2 [
5] while a standard PV module weighs 12–16 kg/m
2 (a glass-glass module 14–17 kg/m
2), and with racking a total weight can exceed 40 kg/m
2 [
6,
7]. Additionally, old buildings erected in the People’s Republic of Poland may be unsuitable for solar system due to past regulations. Snowfall is the main factor that contributes to a building’s roof load bearing capacity. In Poland until 2006 the time during which the value of a given load should not be exceeded was only 5 years [
8]. The amendment of the standard PN-EN 1991-1-3: 2005 Eurocode 1 has extended the snow load return period to 50 years. This resulted in an increase in the required snow loads and hence load bearing capacity. In the case of some buildings existing before the amendment, a recalculation of this parameter reduced their capacity significantly.
Nevertheless, there are four possible solutions to install the PV system anyway. One can utilize a low-ballast or aeronautical mounting system, but still the maximum permitted load may be exceeded because of the standard PV module weight [
9,
10]. The second option, more suitable for the old buildings, is a customized steel construction which transfers the load from the roof to walls. However, the cost of such a construction usually makes the investment unprofitable. Alternatively, the roofing can be replaced by building integrated photovoltaics (BIPV), but again it costs a lot. Furthermore, BIPV generates an excessive load which may not be well tolerated by older buildings’ structures [
11]. Therefore, BIPV is a suitable choice only for newly built facilities. Finally, lightweight PV modules can be mounted. They usually weigh up to 7 kg/m
2 [
3] and do not require a racking system. This makes them suitable for even very low load roofs. It is worth mentioning that the main difference between BIPV and lightweight PV modules lies in their relation with a building. While BIPV replaces conventional materials in parts of the building envelope and cannot be removed without impact on the structural integrity, lightweight modules may be mounted using assembly like standard PV modules or directly fastened onto building construction materials using for instance tape. Hence, in the second case they can be considered as building attached photovoltaics (BAPV) [
12].
1.1. State of the Art
The majority of lightweight modules consist of a polymeric sheet on both sides of encapsulated thin-film PV cells [
11]. In some cases, aluminum or stainless steel is used as a backsheet [
13,
14]. Owing to the fact that glass and frame contribute about 69% and 11% to the weight of a conventional PV module [
15], respectively, removing these components allows achieving a load as low as 2.7 kg/m
2 [
3]. In addition, using thin-film cell like amorphous or microcrystalline silicon provides flexibility to a module, which is desired, for instance, in military or transport applications. However, full dimension thin-film modules usually do not achieve efficiency higher than 10% [
14,
16,
17]. Only CIGS modules have higher efficiency, but it is still lower than crystalline silicon modules [
18]. Therefore, it seems reasonable to utilize monocrystalline silicon cells to fabricate a high efficiency lightweight module. Nevertheless, CIGS cells indicate many advantages in case of BIPV [
19].
There are some commercial silicon lightweight modules, but their quality is questionable [
3,
20], even when they claim to comply with the IEC or UL certification. Therefore, in the literature some prototype solutions have been proposed.
A.C. Martins et al. [
3] have introduced a glass-free lightweight module (about 5 kg/m
2) which was investigated by the IEC 61215-2:2016 hail test. The module structure consists of a composite sandwich backsheet (with an aluminum honeycomb core) and polymeric frontsheet with grade of glass scrim. The researchers examined the impact of backsheet bending stiffness and frontsheet thickness on the module resistance. It turned out that the smaller backsheet stiffness and thicker frontsheet, the higher module hail resistance. Later, the module structure has been slightly changed which resulted in higher weight (about 6 kg/m
2). In a similar paper, a detailed manufacturing process of the module was described [
21]. The device has undergone further tests. It successfully passed damp heat and thermal cycling tests [
20] and also hail and mechanical load tests [
11] as prescribed in the IEC 61215-2:2016. In the next paper [
22] the researchers have investigated different frontsheet encapsulants to improve resistance of their module. After a series of the following tests: ultraviolet-visible, spectroscopy, differential scanning calorimetry, rheology, peel and tensile tests and hail test on mini modules, TPO (thermoplastic) and POE (elastomeric polyolefin) turned out to be the best composition. Subsequently, the authors proposed a mounting system for lightweight modules based on Velcro tape (placed along edges and diagonals) which complies with the static mechanical load test presented in the IEC 61215 [
23].
In [
24] the authors have fabricated and tested mini modules (1- and 4-cell) covered on both sides with an acrylic-film. It allowed to reduce the weight of the structure to 46% of a conventional module weight. The samples have met requirements of the IEC 61215 standard in case of the thermal cycling, humidity-freeze and damp heat tests, and also showed no changes in the PID test.
A different approach to lightweight module construction involves adding a lattice-like structure at the rear side [
25]. Because of this, very thin laminates can be used without losing sufficient mechanical stiffness and hence a weight reduction to less than 6.5 kg/m
2 may be achieved. The authors claim that their module is able to successfully pass the IEC 61215 standard mechanical load test.
Another concept of lightweight module structure was presented in [
26]. Use of a polyester foam as a backsheet allowed to manufacture a full-dimension (60-cell) rigid module weighing 5.5 kg. Its power was equal to 256 kWp, which gives a Power-to-Weight ratio of about 46.6 Wp/kg. After a few months of outdoor monitoring no degradation or deterioration was observed. Cost of the module manufacturing is similar to a standard module. However, a full IEC test sequence has not been so far conducted.
To better evaluate lightweight modules in terms of their weight and area, S. Schindler et al. [
27] have proposed novel indicators. The following parameters were suggested:
Power-to-Weight Ratio P2W [W/kg];
Area density AD [kg/m2];
Weight-to-Power Ratio W2P [kg/W];
Efficiency-to-Weight Ratio E2W [%/kg].
According to the authors’ analyses, a typical silicon lightweight module has P2W ratio equal to around 40 W/kg, while standard modules are in the range of 10–20 W/kg and thin film devices are below 10 W/kg. The best lightweight module has 60 W/kg. The usual E2W ratio of this module type varies between 1.5 and 3%/kg, reaching in one case 12%/kg. However, this indicator is misleading if we consider modules of different cell number (and hence size), and only efficiency should be compared instead. In case of lightweight modules, the values range from 12% to 21% with average 15%. When it comes to AD, the value for lightweight modules is below 5 kg/m2. While P2W and E2W ratios are better for lightweight modules than for standard ones, costs per m2 are significantly higher and more diversified. It varies from 60 to 110 €/m2. In comparison, standard modules cost between 40 and 60 €/m2.
In light of the above analyses, a module presented in [
28] is characterized by a very high
P2W indicator equal 130 W/kg. Due to utilizing a thinner HJT (heterojunction) cells, the module has gained high power and flexibility. The authors also claim that it is possible to achieve 250 W/kg. The module was implemented in different applications like boats and tents. However, it has not been tested according to any standard.