Efficiency: to much of a good thing

Efficiency is a concept that all people, not just engineers, grasp onto: an increase in efficiency is always good. While I have no issue with the general concept, it is dangerous oversimplification that implies efficiency is somehow a better metric. In reality, efficiency is only the easy metric, not the best metric and an over reliance on efficiency to sort technology into good and bad ideas leads to higher overall cost solutions and extreme waste. The problem is one of path functions. When a technology is created it starts somewhere on its cost-efficiency curve based on the constraints of the day. It is improved from there, at an increased cost, until we have a large understood technical space built up around the technology. This works great when constraints stay the same, but when the constraints change, the technology is now up a curve without a paddle. 

This issue is most pronounced in energy technologies, where efficiency is THE metric. Thermal heat pumps are a great example of this. Thermal heat pumps like absorption and adsorption chillers are very high up their technology curves with well understood technical spaces filled with publications, prototypes, and commercial systems. These technologies have always struggled to compete with electrically driven vapor compression systems, but have been heavily invested in all the same. Since 2000, a tremendous amount of thermal heat pump work has gone into increasing their viability for renewable refrigeration applications with the idea that, since we already understand the performance of these technologies, adapting them would be easier then inventing a completely new way of providing renewable refrigeration.

Here is the rub, thermal heat pumps were invented under very different constraints than they are under today. Namely, they were invented as a way of converting some form of burning fuel to refrigeration. Under this constraint the biggest cost of the system is the fuel, thus efficiency at high temperatures is critical. 

Now, these technologies are being re-purposed for applications fed by solar sources. While it is easy to design a solar collector that can generate the temperatures these cycles are used too, it is not cheap. Furthermore the solar "fuel" is free. Putting these two together, efficiency becomes a second order factor and the up front cost because the main driver.

Seeing this issue, several companies are already trying to develop ultra low-cost solar collectors and less-efficient lower cost absorption chillers. But these are stop-gap measures. The real issue is that we have invested a huge amount in technology that was conceived to solve a dramatically different problem. 

In these types of situations, it is very hard to go back to the drawing board. There is already a technology with so much development behind it that it can be difficult to convince anyone to fund you to do something else. What's worse is that, in most cases, the technology that really needs to be developed has a lower efficiency. There is nothing harder than convincing someone that a new technology can be better with a lower efficiency. 

This over-reliance on efficiency as a metric makes the technology of the 20th century, which was developed in under constraints heavily biased to efficiency, creates a significant amount of waste today. Take the CSP vs PV battle of 2000-2010. Even today, CSP maintains a solar-electric conversion efficiency significantly higher than PV, but PV has clearly beaten CSP as the way we turn photons into electrons. If PV and CSP had been compared via a more relevant metric in 2000, perhaps a significant amount of public and private investment money could have gone towards more productive research. 

Moving forward, lets lay off the efficiency talk and look at other metrics! There are a lot to choose from.