E.3 Component Selection

In order to be able to build prototypes, a certain system setup must be chosen and each system component must be dimensioned. The following section addresses the selection of an adequate battery and PV type, solar cell incorporation, and charge controller options.

E.3.1 Battery Unit

First, typical battery parameters such as energy density (Wh/kg and Wh/cm³), cycle life, charging time, cell voltage, discharge rate, operation temperature, and so on were evaluated. This narrowed the possible choices down to the following battery types: nickel–cadmium (NiCd), nickel metal hydride (NiMH), lithium-ion (Li-ion), sealed lead–acid (SLA), and rechargeable alkaline manganese (RAM).

Each technology was assessed on five aspects: cost, efficiency, design, durability, and environmental aspects. The performance of specific battery technologies was categorized as very bad (−−), bad (−), good (+), and very good (++) in the SPM product case (Figure E.3.1). Categorization took into account information from handbooks as well as the particular expertise of the involved PV and industrial product designs experts. From this, only two types of batteries were considered further, the NiMH and the Li-ion, as these did not have a very negative score in any category (NiMH) or fell short only on cost and durability grounds (Li-ion). Finally, the choice was made for NiMH batteries for various reasons. NiMH batteries are available in different shapes and capacities, and voltages are multiples of 1.2 V, which allows a lot of freedom regarding system design. Furthermore, NiMH battery types can be found in electronic retailers or supermarkets, making it easy for the user to replace these secondary batteries if required. In addition, NiMH batteries with almost no self-discharge are available.

Figure E.3.1 Assessment of different battery types for use in the SPM

E.3.2 PV Cell

It is often argued that thin-film PV technology, especially hydrogenated amorphous silicon (a-Si:H)–based solar cells, suits PV-powered products better than crystalline silicon cells. Indeed, the a-Si:H cell type shows outstanding low-light performance, and the spectral response (SR) matches indoor irradiance spectra very well (Reich et al., 2005). Especially advantageous is the fact that the SR of a-Si:H cells almost matches the photopic response curve of the human eye, which makes a-Si:H cells highly effective at energy-efficient artificial-lighting conditions. However, under typical fluorescence tube spectral intensities of 100–150 Lux, the charge demands of the SPM simply cannot be met. Thus, the SPM requires solar energy as an irradiance source, because the energy flux of solar radiation is orders of magnitude higher than that of (energy-efficient) artificial lighting. Especially if the SPM is placed at or close to a windowsill, rather high irradiance intensities will be available for charge generation. Solar cell efficiencies at higher irradiance levels (i.e., 100–1000 W/m²) are generally much better for mono- or multicrystalline silicon (c-Si and mc-Si) than for a-Si:H solar cells. Commercially available c-Si cells have greater than 20% efficiency at Standard Test Conditions (STC), outperforming a-Si:H cells with roughly 7–10% STC efficiency by a factor of 2–3. We thus opted for crystalline silicon–based cells.

E.3.3 Encasing

Placing the solar cell underneath a double-bent, transparent plastic cover within the SPM encasing reduces irradiance intensity, which in addition is unequally distributed across the solar cell due to shading effects. Interestingly, the product encasings' shading effect is slightly overcompensated by optical concentration, as calculated by the software tool 3D-PV (Reinders, 2007). Since the 3D-PV tool could not calculate double-bent interface characteristics, however, this does not include transparent cover transmission characteristics, which we estimated together with transmission characteristics of a two-paned window-glazing system using Fresnel equations in a separate model. Here, we found PV module tilts (i.e., the tilt of the solar cell within the SPM encasing) of 20–30° to be optimal. In the final concept, the solar cell is tilted only 10°, due to a trade-off between the SPM's ergonomics and the required space for electronics, batteries, and mechanics. This permits relatively large cells to be incorporated without sacrificing too much internal space. In order to improve light-harvesting properties, support structures should be designed for fixed and secured positioning of SPMs with about a 45° tilt angle when placed for a sunbath at a windowsill.

E.3.4 Charge Controller

The most simple charge controller would require only a single diode, preventing battery discharge over the solar cell. Overcharge protection could already be achieved by just another diode, which short-circuits the solar cell above a certain voltage threshold. Another option would be a self-regulating system design by matching maximum solar cell voltage to the maximum voltage of the storage unit. This would be desirable, owing to the sheer simplicity of the concept. However, matching battery and PV voltage is difficult, because battery voltage depends on the battery state of charge, and PV voltage depends on irradiance intensity. Voltage converters, in contrast, require rather complex electronics but also lead to greater design freedom. Theoretically, when incorporating a voltage stepping unit, all combinations for the three categories of storage, module setup, and PV technology become possible, as depicted in Figure E.3.2. Voltage converters apply DC/DC up- or down-conversion (of PV voltage), so they are most logically considered in combination with maximum power point tracking. With the single-cell concept already chosen, we opted for charge controller electronics that perform voltage up-conversion and maximum power point tracking.

Figure E.3.2 Morphological chart of possible SPM designs concerning solar cell implementation and the possible voltage ranges of the different subcomponents

On top of charge controlling, the electronics should provide a battery status indication, as defined in the design criteria. It was decided to use only battery voltage as the battery SOC indication. In this case, the selected NiMH battery type only allows indicating battery SOC as either full or almost empty, due to the rather SOC-independent voltage potential of NiMH batteries. It would be desirable to indicate battery SOC through a bar graph composed of four or more elements. Moreover, indicating the positive influence of sunbathing may encourage users to sunbathe the mouse more regularly. However, we opted for a relatively simple but cheap and fast solution.