The Project: Sun Coffee

When I was young, “Recycle Rex” exhorted us to “recycle, reduce, reuse, and close the loop!” Who knew that what teenage me thought was hokey and stupid would find such resonance in later life. To this day I cringe at throwing things in the trash and try to reuse things whenever possible.

My fiancee and I drink a lot of coffee. Each day we make at least 10 cups – leaving a significant amount of grounds behind. While we recycle the waste water into our edible garden, and compost our grounds, we wanted to find ways to further reduce the carbon imprint of our coffee habit.

Previously we’ve used old grounds mixed with sugar as topping on our Rowan’s Royale Coffee Kisses (recipe opens in new window), which gave me an idea. Why not dry my used coffee grounds for later use?

The obvious questions arise: what can old grounds be used for, and, does this actually reduce our carbon usage?

When making the coffee kisses, we dried the grounds in the oven which presents a carbon cost that likely outsizes any savings reuse can present. Especially given that we are reusing an organic product which sequesters carbon as it grows, burning fuel to dry it will just add more carbon to the imprint of using it (harvest, transport, etc.).

To address the first question, this time we made a sun tea from the dry grounds.

The Method

Coffee Grounds Drying in a Cake Pan

To address the second question above, we aired the grounds outside. The combination of dry air and sunlight took the moisture in just a few hours, leaving a fine, dry powder for storage and use.

Making sun coffee presented several additional questions. What grounds::water ratio will provide the best result? How much extraction can we expect from sunlight and at what rate? What taste profile can we expect?

A back of the envelope calculation shows that we usually use about 43-50 grams of coffee per 1.9 litres of water in our Bona Vita drip coffee maker. Because we were reusing grounds, we decided to try a 2:1 ratio per normal unit volumes. We scheduled the brew for four hours in order to hit our target of a reasonably strong, light coffee flavored tea. We expected it would taste like coffee, just diluted (we were wrong, see below).

To do the brew we made a strainer from unbleached cheese cloth and the biggest mason jar we have, which was about 150 ml smaller in volume than the target above. Ultimately we brewed about 98 grams of coffee in 1.75 L of water, in direct sunlight.


The end result was pretty good. We were wrong about some basic assumptions and the flavor was surprisingly different than we expected – in a good way.

Firstly, judging by color alone, we could have limited brew time to two hours instead of four. There did not seem to be any additional extraction from the last two hours of steeping.

Secondly, it seems that the upper limit of extraction was not the measure of grounds but rather the energy transfer possible from the sun. Given that the brew did not get stronger after hour two, but the grounds still had a potent flavor, we think it might be that we could get the same result with fewer grounds – more experimentation is needed.

But what about the most important aspect, the taste? With that we were totally wrong. We expected coffee flavor but diluted and what we received was anything but. The sun coffee has a light, nutty flavor with a beautiful Amaretto sweetness. It is not unlike almond tea, but with a decided hint of fine tobacco. It’s extremely light. A little agave wrecked the first glass by blowing out all the flavor. For the second glass I took a slice of lime and squeezed a little in. The lime enhanced the sweetness of the brew and cut down the tobacco flavor allowing the round almond flavor to shine.

The finished sun coffee brew - light and refreshing when served with a lime wedge.
Cheers to experimentation!

The brew was tasty and refreshing on a hot day. I’d happily have a glass while gardening…or turning over my coffee grounds compost bin!