Industrial Design of a Home Solar Energy System

February 2018   Bianchini

The MIT Development Lab (D-Lab) connects student teams with international community partners who work on projects to better developing communities. Through a D-Lab class, I partnered with Voya Sol, a company that develops home solar energy systems for rural Zimbabweans to build their own micro grids. I and my team focused on the industrial design of the appliance, including the user interface and features that would define how the appliance lives in the home.

Overall architecture and motivation

The elementary Voya Sol solution to the electricity shortage in rural Zimbabwe begins with a box, to which users can hook up solar panels to capture energy from the sun, plug in devices to make use of that energy, and connect an external battery to keep an overflow reserve.  While the solar panels, user devices, and external batteries would be made from a third party vendor, Voya Sol is designing the box itself.  The user interface is where my team and I came in.  We addressed the following question:

• How might we design the Voya Sol box such that it is very intuitive for someone who cannot read to connect and scale the system?

D-Lab motivation

The MIT D-Lab works with developing communities in order to help drive sustainable human development, which should not be confused with sustainable environmental development.  The following figure can help clarify this distinction.  Ben Linder of Olin College first introduced me to this material in a guest lecture.

Human Development Index versus Ecological Footprints by country.  [Source]

The x-axis plots human development index (HDI), for which above a 0.8 is considered to be high human development.  This index takes into account basic health like mortality and morbidity, education like literacy and rates of school completion, and economics like income relative to purchasing power.  The y-axis plots ecological footprint per capita, under which ~1.8 global hectares would yield equal environmental allocation for every person.

This means that for a country to provide a good quality of life as well as be environmentally sustainable, it must fall in the blue region at the bottom right corner of the graph.  The only dot in that region (in 2003) was Cuba.  Countries that are above the ecological footprint maximum but above the HDI minimum, like the US, need to put efforts towards environmental development:  maintaining a high HDI while lowering economic impact.  Countries that are behind the human development index minimum typically are already under the maximum ecological footprint, so they need to put efforts towards sustainable human development:  maintaining a low ecological footprint while raising their HDI.  This sustainable human development is where the MIT D-Lab aims to assist communities in developing nations.  This distinction is deeply engrained into the D-Lab projects and is why I performed a life cycle analysis for my developed product.

Specifications

The specifications which we used to define the box's scale, proportions, and functionality were given to us from Voya Sol themselves.

Table 1:  Specifications that defined the industrial design of the Voya Sol box.

The most challenging part of the design would be that illiterate users would be interacting with it.  Another driving factor for the design was the style of house typical in the rural villages.  It is common for these to be single rooms for the entire household, as well as mud floors with common flooding outside and inside during the wet season.  Additionally, the boxes should ideally be able to be repaired locally.  And lastly, families who were to own systems like this would want it to be a beautiful object in their house since it would be a status symbol.  These additional factors drove the design further:

• I took to using symbols that are common on cell phone interfaces used in the region, in order to draw upon already associated visual cues to give illiterate users a sense of how their box works and is performing.
• Any lights on the box to indicate the box's status might become bothersome at night if people are trying to sleep in the same room.  Thus I developed a folding panel that would obscure the lights when in the closed configuration.
• To avoid electrical components from interacting with flood waters, I incorporated a dual-design for both sitting on its own feet on the floor or shelf, as well as a built-in wall mount if possible.
• I limited the visual indicators to be just colored LEDs, inspired by the need for the box to be repaired locally.  Other possible solutions included a screen, though we determined that the availability of LEDs was much more reliable in these remote villages than would any kind of screen.
• The box had to be visually appealing in addition to functional.  Colors are important, as wildly colorful and patterned textiles in Zimbabwe are commonplace and highly regarded.

Initial design

I drew the original design, from which our final proposal was derived.  I incorporated feet as well as a hanging mount, bright color-coded bands, lights on the front face with connectors on the sides, and icons to give users indications of how the box is working.

To improve the design above, I devised a front panel that could swing closed to keep the icons hidden or swing open to reveal the front and/or form a shelf to hold charging objects.

Initial prototype

I built the prototype by laser cutting and engraving acrylic panels, attaching them with hot glue (meant to be temporary if the boxes had to be transported to Zimbabwe for testing), and applying color by using marker on engraved surfaces.  I incorporated puzzle-piece-like tabs for the pieces to fit together properly.  While this would not be the manufacturing method for large scale manufacturing, this was a quick, high-fidelity prototyping method that gave me results incredibly similar to my initial drawing on paper.

Two of the laser cut panels, before coloring in most of them.  I engraved all of the graphics that would keep some color so that I could use marker to color it in.  The rough, engraved surface traps the ink while the excess ink on the smooth acrylic wipes right off.  This was a quick, effective prototyping method.

I inserted actual connectors that would be used in the final device to give the prototype a full looks-like presentation.  The black connectors are MC4, which are common for solar panel connectors, and the lower ones are USB ports.

The final colored front panel showed indicators for the external and internal batteries (orange), the solar panels (yellow), and the users' charging devices (blue).

Feedback from first prototype

After making our first prototype, we presented our progress to our partners at Voya Sol as well as to our class and other invited guests.  We gathered a lot of feedback for changes to make for our second iteration:

• Despite being given the exact size of the box initially, our prototype helped demonstrate the scale of the box.  Therefore, our Voya Sol partners decided making the box smaller would be an important feature.  The next box was to be < 50% of the volume of the first.
• The box could use an overall power switch to shut down the whole system.  Additionally, it would be beneficial to be able to shut off the LED indicators while still keeping the system running, so we would include an indicator button to toggle between on and off.
• White is a color that could get dirty easily, so our Voya Sol partners drew upon Zimbabwean textiles to give us three different color variations to use.
• Our Voya Sol partners provided us with their updated logo, which they wanted to feature large on the front folding panel and again smaller on the front indicator panel.
• I decided to use a different battery icon to provide a distinction between the charging devices and the batteries themselves.
• The final product would have to expel a significant amount of heat from inside the device, so we were to incorporate ventilation holes somewhere in our second design.
• We had not yet specified a connector for the external battery.  After doing some research, we found a round connector common for battery hookups.  We would incorporate that connector into our second iteration.
• Placing the strings to hold up the shelf in the first iteration was a bit of an after-thought, so I would build in tie points into the panels for the second iteration.
• Other members on my team had been working on how the electronics for lighting up the LED indicators could work using an Arduino.  The second iteration would incorporate a functional circuit to light up the LEDs into the next box design.
• I thought there could be some improvements on how the user opens and closes the folding panel, namely by including a tab on the folding panel and a corresponding cutout in the top panel.

The colors for the next iteration would keep standardized color-coding for the connectors while changing up the box and lid colors.

Second design

The second design incorporated an overall smaller architecture, featuring ventilation holes on the sides, three different color schemes (only one pictured), updated icons, a new battery connector (not shown), a hole for an indicator toggle button, a shut-off switch (not shown), and the updated Voya Sol logo (not shown).

Each box was made out of 7 panels.  The large hole in the bottom panel, which is the back panel, is for wires for the electronics, just for prototyping purposes.

Second prototype

I made the second series of prototypes using mostly the same methods as the initial prototype.  The major difference in prototyping methods involved using vinyl stickers instead of markers to color the indicator bands.  This provided a sharper color contrast and smoother finish.

Laser cutting the panels for three boxes!

Applying the vinyl stickers allowed for much crisper color contrast.  As you can see for the device bands, which had not yet had vinyl applied, at the bottom of the panel, I laser engraved the outlines of the bands to make alignment for the vinyl stickers easier.

The final prototypes came out clean and visually appealing as well as demonstrated our incorporation of suggestions from our previous design review.

We made the red box demonstrate the full electronic capability as well as a looks-like for the industrial design of the appliance.

All of our prototypes together.  Our very first prototype was a cardboard box, in the back.

This photo gives a sense of scale of the appliance.

Life cycle analysis

A product life cycle analysis assesses a product's environmental impact from its manufacturing through its use and eventual disposal/recycling.  Because this course was in sustainable human development, considering environmental impact was crucial.

Table 2:  Calculates the total energy (in MJ as well as in kg of carbon per kg of the product itself) that goes into one of our early Voya Sol box designs. Subsequent designs eliminated the hard rubber feet/casing, however the life cycle analysis will eventually show that this design difference is negligible.

Table 3:  Calculates the total energy (in MJ) that the solar panel can generate per day.

Table 4:  Calculates the total carbon equivalent (energy) per day that kerosene would contribute in order to provide the same amount of energy as the solar panel.

The Voya Sol box enables users to capture the energy produced by the sun in order to charge their home devices.  The total system that requires sunken environmental manufacturing costs is the solar panel plus the Voya Sol box.  Since at the point of doing this life cycle analysis we didn't have detailed information about how the Voya Sol box will be made or what solar panel will be used, we estimated energy costs for the Voya Sol box by making a bill of materials modeled after a similar product in the market space.

Our analysis led us to a box with sunken environmental material costs of ~745 MJ, or 26.36 kg C (Table 2).  The Voya Sol box can be used with up to four 10W solar panels. We performed these calculations assuming only one 10W solar panel was used.  We assumed that the solar panel can generate 10W for 8 hours per day (equating to 288 MJ/day), and that if not for the solar panel, the users would have otherwise been generating that energy by burning a kerosene fuel (equating to 5.78 kg C / day) (Tables 3 + 4).  These numbers mean:

1. Just in terms of the energy saved by eliminating the fuel source, the box pays off for its environmental investment in just over 4.5 days.
2. Just in terms of the energy generated by the solar panel and delivered to users’ devices, the box + panel system pays off for the box’s environmental investment in just over 2.5 days.
3. Factoring in both of these energy savings (elimination of using kerosene fuel + the addition of renewable solar energy generated), the box + panel system would pay off for the box’s environmental investment in 1.6 days.

Note that this is just the time for the box + panel to pay off the box, and the system would have to work longer than this to also pay off the environmental costs of the panel itself (Figure 3).  According to this study on solar panel environmental payback periods, the most environmentally efficient solar panels pay off their own environmental debt in no shorter than 1 year of use, and can often take as long as 4 years.  Thus, the Voya Sol box has an insignificant environmental cost in comparison to the solar panel itself with which it would work.

A diagram of how the Voya Sol box intercepts the environmental cost of using a different carbon-based energy source.

According to the same solar panel study referenced above, solar panels typically last ~30 years.  If the panel pays for its own environmental investment in a maximum of 4 years, then at most 13% of its lifetime needs to be devoted towards offsetting its own environmental investment, and 87% can be used to offset the cost of the Voya Sol box(es).  87% of the total energy the solar panel would generate in its lifetime (30 years, generating 288 MJ/day = 3,153,600 MJ) would be 2,743,632 MJ, or the equivalent of 3,682 Voya Sol boxes. Thus, as long as 3,682 Voya Sol boxes last 30 years, or as long as 1 Voya Sol box lasts at least 3 days, the solar panel + Voya Sol box system will generate more energy in its lifetime than required to offset its environmental investment in material and manufacturing, acting as a 20-80 system.  Since a lifetime of 3 days is incredibly low and the target lifetime of the Voya Sol box would be on the order of years or decades instead of days, this system is undoubtedly well into the 20-80 product zone.

The life cycle analysis from cradle to grave of the Voya Sol box + solar panel system would have total energy costs including sourcing of materials, transportation in between steps, manufacturing steps (which end up being insignificant compared to material costs), and disposal.  According to our sponsor, much of the electronic components will be sourced from China, incurring significant shipping costs to transport those components to the market in Zimbabwe. Ideally, the assembly and other hardware materials (such as box walls and feet) could come from Zimbabwe or closer locations, incurring less significant but still present transportation costs between factories and distribution sites.

We could reduce the impact of the material and manufacturing costs by using recycled materials or materials with low environmental impact whenever possible.  Increasing the durability and life cycle of the box and panels themselves will also reduce disposal-related environmental impact. Furthermore, if materials are recyclable, or the box can be used for other purposes afterward, the materials could be used to offset other environmental costs past the product’s lifetime.

Voya Sol's future with our work

Voya Sol loved our progress throughout the semester, and they brought our prototypes to Zimbabwe in the summer of 2018 to get direct user feedback on them as well.  With any feedback from the users incorporated, our design will get repurposed to suit large-scale manufacturing methods (most likely injection molding) to become Voya Sol's first on-the-market product.

I and my teammates presented to a group of fellow classmates, industry professionals, and academic advisors for our final presentation before delivering our prototypes and supporting materials to our Voya Sol partners.