4. Results and Discussion
4.2.2. Minimum Conditions for Optimal Performance
Regarding a comparison of modular performance, the previous work (Chapter 3) clarifies that such an exercise would not be prudent in light of differences in their optimal range of operational temperatures (Table 2-4). Nevertheless, to determine the minimum requirements that are required to make these modules ecologically effective on all categories, HH module was chosen as a test-case, given its best ecological output among all modules. Two hypothetical scenarios were considered for this module, one each having its conversion efficiency or TCRC kept as constant (the same as for original HH module) and the other parameter allowed to vary in a manner that could help in attaining positive
outcomes on all categories, keeping everything else constant. Remarkably, calculations showed that even at extremely high and impossible levels of conversion efficiency (50 %), or even at TCRC = 0, this hypothetical HH module was incapable of accomplishing this aim (figure not shown). This was solely due to the extreme mismatch between negative effects of production and positive impacts of use for HH on FET category (~ 80 %) (Figures 4.1–4.3), which could not be lowered even with such drastic changes in its conversion efficiency or TCRC. Similar conclusions were obtained for other modules as well.
Subsequently, attempts were made to determine the minimal usage duration for all modules by considering an alternative hypothetical use scenario. Under this scenario, it was assumed that all modules would be recovered and used in as-is state over several peak- load NG power plants that operated under the same condition (4 h/day). An additional assumption was that each module would be used in this manner till its conversion efficiency reached 1 % of its original value due to thermal cycling. However, calculations showed that even under this hypothetical use case, no module exhibited positive impacts on all categories (not shown here), especially on FET and HNT impacts (where no module was seen to be ecofriendly). This was again, a major consequence of considering the substitution of NG-based electricity with that from these modules.
In sum, these findings suggest that even if: (a) Conversion efficiencies are dramatically improved; (b) TEs are used for longer duration over multiple plants of similar nature; and (c) Effect of thermal cycling on their conversion efficiencies are made negligible, existing TEs may still prove inadequate in overcoming negative effects of their production for periodic waste heat emitting applications. This implies that TE devices may
be ecologically beneficial only for such applications where one or more of the following three possibilities are realistically achieved: (a) They are used for much longer duration per cycle; (b) They replace a more polluting form of energy source, such as coal; and/or (c) Their production-related impacts are lowered by a significant degree (by as much as ~ 70- 80 %). Of these, the first two possibilities are entirely dictated by the nature of application, which shows that at least for peak-load NG-based power plants, this platform may not be ecologically suitable for large-scale application. In contrast, an exploration of the third possibility requires focusing on the complex interplay between optimal operational temperature range of modules (which also affects their conversion efficiency) and other factors that influence their life cycle performance, critical among which are three: (a) Mass of module; (b) Nature of constituent elements used in TE systems; and (c) Energy consumed till their production stage. To understand this in some detail, two systems are focused upon: HH (which shows the best performance among all modules if taken at face- value), and SK-2 (which ranks among the worst performers on same ground).
HH benefits from a number of advantages with regard to the aforementioned critical factors. First, it has the lowest mass among all modules (13.38 g/module) barring SC (Table 2-3). Further, even as its output power is the fourth-highest (15.5 W) among all modules, its conversion efficiency is the second-lowest (4.50 %) among all modules (Table 2-4). As a result, it requires the least number of individual modules (among all TE module sets) to convert 1000 W of waste heat into electricity (2.90) (Table 2-4). Also, HH shows the second-lowest amount of GW impact among all modules till the production stage on per- module basis (Table 3-1) – which is a strong indicator of energy consumption, given strong
linkages between both aspects (GW and energy use). Thus, all these factors combine to lower the amount of constituent elements required for HH module set and thereby, its ecological impacts on all categories – lower use of constituents on toxicity- and scarcity- related categories, and lesser energy use on GW and FRS categories.
On the other hand, SK-2 module is seen to exhibit the largest production-related GW impact among all modules (on per-module basis) (Table 3-1), which indicates its high (fossil-based) energy requirement for processing158. This also accounts for its worst performance among all modules on FRS category. In addition, SK-2 also shows the poorest output on four toxicity-related impacts (FET, MET, HCT and HNT) (Figures B.1–B.4, 4.1– 4.3 and Tables B-13–B-24). This is the outcome of its highest per-module mass among all systems (107.44 g/module; Table 2-3) and predominant use of antimony in its TE legs (Table 2-1). As described in Chapter 3, antimony is hazardous for environment on these categories via toxic sulfidic tailings emitted during extraction and processing143,145–147,158.
However, on both TET and MRS, SK-2 performs better due to respective use of (toxic) tellurium in BT-1 module and scarce germanium in SC module142,143,145–147,155,158.
This complex interplay between various factors encompassing production and use stages, implies the need for major advancements in developing novel TEs that: (a) Exhibit higher conversion efficiencies; (b) Use elements which are non-toxic by themselves and also help to avoid toxic waste emissions during their extraction and refining; and (c) Are produced using techniques that are efficient in their energy consumption. Any endeavor towards simultaneously addressing all these concerns will, however, be a highly onerous and challenging task to accomplish, as it would necessitate the involvement of multiple
stakeholders, including researchers, potential end-users, TE module manufacturers, and even policymakers for commercialization of such systems.