Christopher X J. Jensen
Associate Professor, Pratt Institute

Biophotovoltaics: a promising design innovation, or a great example of lack of quantitative design?

Posted 27 May 2016 / 0
2016-05-27aMoss image courtesy Dick Mudde via Wikimedia Commons

GreenFabLab Barcelona Moss Voltaics

A student in my Ecology for Architects class pointed me towards this design project, which creates a building facade system designed to pull electrical current from small growth chambers containing moss. The technology — dubbed a biophotovoltaic — turns the energy captured by photosynthesis into current. I was a bit skeptical about how this was even possible; it has been a few years since I took my plant physiology course, but I remember that the chloroplast is a complex little photosynthetic powerhouse, and chloroplasts are numerous and well-distributed across plant cells (and therefore tissues). For that reason it is hard to imagine directly “tapping into” the photosynthetic process to generate electricity. Instead, these systems utilize a rather indirect byproduct of photosynthesis to generate electricity. As described, it appears that each of the moss chambers is able to collect stray electrons produced by the respiration of soil microbes. What’s the connection to the moss? Well, the soil microbes are breaking down organic compounds (obviously carbohydrates or compounds derived from carbohydrates) produced by the moss. It’s rather surprising that this indirect source of electricity can be tapped at all, and so realizing that it can be is cool-gee-whiz moment. That said, I am always suspicious of cool-gee-whiz moments: is this potential source of solar power substantive, or just spectacular?

As the blog post describing the project makes clear, many aspects of this design are very nicely realized. The actual chambers are designed to assure that the mosses — which are shade-loving plants — don’t get too much direct sunlight. The design team is to be commended for rigorously testing their creation: they monitored the humidity, temperature, light intensity, and — most importantly — voltage produced in experimental moss growth chambers. But if you spend some time really analyzing the description of this project, one thing becomes pretty clear: the focus here is on the design problem of making these chambers function, not whether that function would provide a practical source of solar power.

Here’s my worry with this biophotovoltaic concept: the plants involved perform so little photosynthesis and the means of harvesting energy from that photosynthesis is so indirect that any impact these kinds of systems offset won’t be worth the impact of their manufacture, installation, maintenance, and eventual disposal. As I have emphasized before, for any design to be considered “sustainable”, it must perform quantitatively. To really understand whether this design idea could ever be implemented in a way that was environmentally beneficial, we need to compare the impacts it creates with the impacts it might offset. To really get at the impacts created by the design, one would have to perform a Life Cycle Analysis of all of its components; without actually performing such an analysis we can see that there would be impacts from extracting materials to make the chambers, from actually firing & glazing the ceramic chambers, and from the installation and maintenance of the biophotovoltaic system. To get at the impacts this design might offset, we need to look at the electricity that the system might capture for use, as this energy would offset sources of electricity — such as our chief source of electrical power, coal-fired power plants — that produce significant impacts. Although there’s not enough information in this post to really calculate how much impact they might offset, a couple of indirect signs point to this design being incredibly low-output.

I confess that I am no expert in electronics, so if anyone detects any faulty logic in the rough calculations I make below, please make an informative comment at the bottom of this post. According to this post, each of these cells — which occupy about 0.010 m2 of facade surface area — outputs up to 0.50 volts. This is comparable to a typical photovoltaic panel cell (such as this one), which outputs about 0.17 volts per cell occupying about 0.008 mof surface area. But while voltage per area is important when considering the potential of a solar electric system, what really matters is wattage: what the actual usable electrical output of the system is. Unfortunately, this biophotovoltaic post doesn’t indicate how many amps are put out by this system, and without knowing the amps you can’t figure out the wattage output of this system. I suspect that this is the Achilles’ heel of this system: it probably puts out very low wattage. This photovoltaic panel is about 0.59 m2 in area and at peak puts out 80 watts at 12 volts. This is more than enough to power a typical laptop computer, which uses about 60 watts. A comparable moss-based biophotovoltaic could put out the same voltage using a smaller surface area: about 0.24 m2. But that doesn’t mean that this biophotovoltaic system would put out enough wattage, and considering the very indirect source of electrical current it harvests, I very strongly suspect it would not. This diagram — featured in the post on this biophotovoltaic system — seems to confirm the quantitative logic I employ above: the surface area needed to provide electricity for a laptop would be a whopping 8.00 m2! That’s roughly 18 times the surface area of a photovoltaic panel performing the same function. If you want to build a building that employs this sort of biophotovoltaic systems, it had better have a lot of surface area for these little moss chambers and have extremely low energy requirements!

It’s an interesting question whether the employment of these biologically-composed photovoltaic systems might have lower impact than employing a photovoltaic panel. The only way to really know is to perform Life Cycle Analysis on each design. I know that there are a lot of materials that go into the creation of a conventional photovoltaic panel, and I imagine that the extraction, refinement, and manufacture of these materials creates substantial impacts. But the maintenance on a photovoltaic cell has to be pretty low. It would be interesting to see if the manufacture of these biophotovoltaics would create less impact than creating a comparable photovoltaic panel, but as discussed the difference in overall surface area required would lead one to guess that the biophotovoltaics might have comparable impacts. And then there’s the maintenance: although the design of these biophotovoltaic chambers seems really smart to me (for example, they use a hydrogel which retains water, making watering less necessary), it is hard to imagine that over a similar lifespan these living systems wouldn’t require a lot more maintenance than a photovoltaic. It would be fascinating to see how my intuition — which I would be the first to admit is pretty worthless — holds up to actual Life Cycle Analysis.

I can’t say with certainty that this design project is a sustainability bust, but it sure seems like it might be. Like a lot of sustainable designs, I think that it could be a failure because it leads with the cool-gee-whiz factor rather than following where the actual design problem at hand leads. Electricity from plants? That’s so cool! Sure, it’s cool, but is it practical? The design problem this project has to tackle is how can we efficiently replace the impacts of more damaging sources of electricityBut this project seems to be mostly about how can we get moss that produces electricity to grow on a building facade? Lost is the critical goal — efficiency of electricity production — in the actual problem this design tackles. And perhaps that’s not a disaster if this design is a stepping-stone to a more efficient design. But before you go figuring out the optimal angles for chambers designed to grow electricity-generating moss on a building facade, it makes sense to make a simple calculation: how much electricity per area can moss-based biophotovoltaic systems generate, and how does that compare to existing technologies?

Thanks to Kat Donnelly for bringing this intriguing project to my attention!

A Minor Post, Biodiversity Loss, Climate Change, Green Design, MSCI-271, Ecology for Architects, Quantitative Analysis, Science in Art & Design, Sustainability, Sustainable Energy, Sustainable Urban Design, Web

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