原创 When a worthy design priority yields unpleasant results

Energy efficiency is an important design consideration these days, and why not? Using less is a good idea, all other things being equal.

But all other things are often not equal. In the past few years, we've replaced a dishwasher and washing machine, and are now looking for a refrigerator, after their predecessors served long and well. All the replacements are far more efficient (electricity and water) than the existing units, which should be good.

But this also has its downsides. The dishwasher, for example, takes at least 70 minutes for a regular load, while the unit it replaced needed only 35 minutes; the front-loading washing machine also has a much-longer cycle time than the top-loader it replaced.

At first, I thought the dishwasher cycle time was unique to our model, but I did some research and it turns out that nearly all dishwashers now are this way: they have traded cycle time for efficiency. I also found out that the new refrigerators save energy both by using a more-efficient motor/compressor, of course, but also apparently by using thicker walls, which then reduces their internal volume significantly (their external sizes are often constrained by the need to fit into existing kitchen cabinet cut-outs, in replacement situations).

So there's the dilemma when an overriding design goal—whether set by regulations, standards, new rules, or trends—has consequences or downsides that are ignored or glossed over. I understand that there are some extreme-design situations where there is no choice, and one design imperative truly carries more importance than others. For example, for an ultra-low-power, remote-monitoring datalogger which must run for years on a single battery or merely harvested energy, dissipation is far, far more important than size.

But in appliances, it would be nice to have a choice, where I might choose a little less efficiency in exchange for shorter run times. After all, real design is about making tradeoffs and managing the weightings among various conflicting priorities; the same guideline applies to purchasing decisions. As the saying goes, "there is no such thing as a free lunch."

Unfortunately, the laudable goal of energy efficiency seems to be given priority status that other objectives don't merit. As engineers know, the law of unintended consequences often follows close behind, such as more materials needed (those refrigerator walls) or lighter, more fragile structures employed (an automobile chassis).

Have you had design situations where a single goal—besides low cost—strongly dominated your project? Or where the downside of this was either ignored, or not realized until much later?