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On how to roast coffee with solar power

Tracking the sun with mirrors. Solar concentrators are getting plenty of cleantech buzz: they’re low cost and ready to work. We talk to a solar concentrator “inventor” whose backyard creations roast coffee.

The market for solar energy is valued at $14 billion (November 2007) and by 2012 it’s expected to grow to over $50 billion. While silicon-based panels dominate the current market, the search for a lower cost alternative has been dominated by talk of thin film technologies (CIGS, CIS and CdTE), but recently attention has been building for a very old technology.

Since there is a limited amount of energy that can be captured from the sun per square mile, this solar constant can’t be increased and proves a barrier to solar efficiency for conventional photovoltaic technology.

Those investing and applying solar concentrator technology are getting around this barrier by concentrating the sun’s energy with mirrors to create “hundreds of suns.”

Dishes, troughs and towers

Solar concentrators used by the ancient Chinese and touted by Leonardo Da Vinci, but more recently they’ve gained attention as an affordable way to harness the sun’s energy. The technology is simple: multiple mirrors (hundreds or thousands) are used to concentrate, or magnify, sunlight to make it more powerful. There are multiple ways to harness this energy:

  • Direct heat- using a dish, trough or tower system to heat household hot water or even to cook (or roast coffee, for example).
  • Electricity generation- using a dish, trough or tower system to heat some type of liquid (water, oil or other) which generates steam which powers a turbine.
  • Concentrator photovoltaics (CPV)- making photovoltaic cells more efficient by concentrating the sun’s energy on individual cells.

VCs believe in concentrators

Much of the venture capital buzz has been around concentrator photovoltaics. Silicon Valley-based SolFocus had raised 95 million in venture funding by November 2007 without even releasing their product: a pole-mounted concentrator developed in conjunction with Xerox’s Palo Alto Research Center (PARC).

Outside of the valley, plenty of others have received VC money including, Germany’s Concentrix Solar, the UK’s Whitfield Solar, Australia’s Green and Gold Energy (SunBall and SunCube) and Southern California’s Energy Innovations (Sunflower array).

Venture backed Solaria describes the motivation behind the investment in this technology with their question on their website: “How do we make solar energy a mainstream reality?”. Their answer of course is using concentrators to “optimize silicon utilization.”

Breaking the 40% efficiency barrier with hundreds of suns

Rather than improving cell technology to make them more efficient, scientists are using solar concentrators to make the sun more efficient. Those at the Boeing subsidiary Spectrolab broke the 40 percent efficiency barrier for solar cells in December of 2006 by not relying on “one sun insolation”, but by using lenses and mirrors to capture energy from hundreds of suns.

The cells that will drive a new California power plant will be receiving the equivalent of 625 suns, as GreenVolts founder and CEO Bob Cart explained to CNET’s News.com Michael Kanellos. Kanellos claims that the firm’s High Concentration PhotoVoltaic (HCPV) system will drive the “largest concentrator solar plant for utilities in the world.”

In November of 2007, Greenlight Energy Resources led a 10 million dollar round of funding because of confidence that this technology “will enable utility-scale solar near the point of use at competitive prices“, according to Greenlight director Jim Trousdale.

Spain’s solar tower

There are also power plants using solar concentrators that don’t rely on any type of photovoltaics. The 624 mirrors outside of Seville, Spain and the 115 meter high receiver are the main components in Europe’s first commercial concentrating tower. When it opened on March 30, 2007 it began providing power for up to 6,000 homes (11 Megawatts).

Funded partly by European money, PS10 is run by a subsidiary of Spanish energy giant Abengoa (Solucar) and is just the first in a set of solar power plants to be constructed in Sanlúcar la Mayor, Spain. When completed in 2013, the plants will generate more than 300 Megawatts (MW) of energy, enough to power the households of Seville.

Australia’s Big Dish

In Canberra, Australia researchers at the Australian National University (ANU) are using a dish type solar collector. The 400-square meter “Big Dish” is the world’s largest solar concentrator dish.

It resembles a satellite dish covered with mirrors which focus light onto a center receiver where water is super-heated and the steam drives an electricity-producing generator.

The Big Dish can power just 100 homes, but as Dr. Keith Lovegrove – head of the ANU’s Solar Thermal Group- explains, the idea is for hundreds or thousands to be strung together to create power stations large enough to power cities.

Google’s multi-million dollar bet on zero-silicon solar

In November of 2007, Google announced that their well-hyped clean energy initiative would focus on solar thermal technology. In 2008, they will be investing millions in “zero-silicon” solar.

The recipient of much of this investment is Pasadena, California’s eSolar who describe their power systems as using “reflected sunlight as a heat-source to drive electric generators.”

This isn’t just a PR, or greenwashing, move. Google co-founder Larry Page is confident the technology can be competitive on the open market. “Our goal is to produce one gigawatt of renewable energy capacity that is cheaper than coal.  We are optimistic this can be done in years, not decades.”

A concentrator plant from the eighties

Using solar concentrators to create electricity is not a new concept. In 1985, solar-thermal-electric generating system (SEGS) plants in Kramer Junction and Harper Valley, California began providing power using a trough type concentrator to heat oil to create steam to drive a turbine.

Although the plants haven’t received the attention of photovoltaics, according to PARC’s Scott Elrod, these plants have had more of an impact than PV technology. “Altogether, they generate an impressive 354 MW of power, and although no new capacity has been added since 1990, they had produced more solar kilowatt-hours than the total installed capacity of PV in the US by the end of 2004.”

Grid parity

On a smaller scale, concentrators are being created for home use to amplify the power and cut the costs of photovoltaic panels.

Brad Hines from Soliant Energy claims their new modules will cut the cost of solar in half and by 2010 their second-generation design will deliver solar cheaper than many conventional power companies.

Concentrators for backyard mechanics

Because the technology is so simple, during the energy crunch of the 1970s, one American firm developed a device specifically for “the backyard mechanic” that produces enough energy to heat the water in an average home.

Teton Engineering still offers their design online free of charge. It costs about 500 or 600 (1980) dollars to build with parts that can be easily found and assembled by the “average hobbyist”. Once constructed the concentrator will produce 6000 watts of electric heat given unobstructed sunshine which is more than enough to power an electric water heater.

The design may be fairly simple, but the the finished product- the concentrator- is extremely powerful and those at Teton post the following warning with their instructions: “It is experimental, unsafe, and just plain dangerous. You or others may be blinded, maimed or killed while working with this device.”

Solar-roasted coffee

In 2003, one backyard mechanic began building his own concentrators in his parents backyard as a cheap and fossil fuel free way to roast coffee. In 2007, David Hartkop and his brother Mike have turned this concept into a business, selling their coffee across the US (via online sales) and at their Solar Roast Cafe in Pueblo, Colorado. Selling 100% organic and 99% Fair trade coffee, they claim to sell “the most Earth Friendly brew this planet has ever seen.”

David (trained not in engineering, but in special effects) has experimented with every type of solar collector: the dish, trough and tower varieties (on a much smaller scale than Australia’s Big Dish or Spain’s PS10 power tower).

While he does all of his designing and tinkering in his parents Central Point, Oregon backyard, after perfecting the second generation collector, the brothers moved the concentrator- and business- from cloudy southern Oregon to sunny Southern Colorado.

We visited David in Oregon (near the California border) for a tour of his collector graveyard (the Helios 1 and 2 solar roasters) and a glimpse of his newest design (the Helios 4) which will have a power of 70,000 BTU and save over 38,000 lbs/year of carbon dioxide.

faircompanies: Why did you decide to build a solar collector to roast coffee?

David Hartkop: “We started this about four years ago. My brother was living at home and so was I and he’s a coffee roaster by trade and had done an apprenticeship for coffee roasting and I’d done a little engineering projects after school and then I became really interested in solar power. We wanted to come up with something we could do together and this seemed to be the thing that combined our talents; building a big solar concentrator to provide the heat for a coffee roaster which he knew how to use.”

“We needed some kind of coffee roaster and we couldn’t afford to buy one at that point in time so I told them, ‘I can make you a coffee roaster, but we have to improvise’. We had to come up with something we could make and we both decided we wanted to do something environmentally friendly and interesting, some kind of a hook for a company, so we came up with the idea that we would use solar energy to provide the heat for coffee roasting.”

An old satellite becomes a solar concentrator

(David walks over to the back part of his yard where two satellite-dish sized collectors are now sitting in the shade).

“These are the first two solar roasters that we made. This is the very first one; we call it the Helios 1. It actually looks like a satellite dish because it’s made out of an old satellite dish. The framework is the ribs of an old satellite dish. Instead of just covering the dish with a mirror or something shiny we use a number of separate little mirrors that we can align individually; so there are actually little eight by eight inch plastic mirrors all lined up.”

“The overall effect of having all these mirrors is that you can basically focus the sunlight to a target right here and the target was our coffee roaster and it would raise the temperature up to what we need to roast which is usually between 450 and 500 degrees Fahrenheit. The first test we did we were able to heat up aluminum hot enough to melt which is around 1200 degrees Fahrenheit so we knew we’d have more than enough power; we just had to put it in the right place and use it properly to roast the coffee. This whole arm, which we move manually, tips up to follow the sun. This ring casts a shadow on this target so we know that everything is lined up.”

“This little system is made so it can pivot around. I almost burned down our garage with one of these. I had it sitting in front and the mirror was turned one way and the morning light caught it and when I came outside it was just starting to burn the paint off the garage.”

A big aluminum marshmellow and glass mirrors vaporizes coffee

(David walks over to what looks like a box wrapped in aluminum foil which is connected to the arm of the other dish).

“This right here is the very first coffee roaster that we made; it’s kind of like a big aluminum marshmallow at this point. It has all the parts that a coffee roaster needs; it’s just kind of low tech compared to the newer one. There are a series of motors that would turn the drum over and over, like a clothes dryer, and the motors are powered by a solar panel that was separate.”

“The electricity was coming from a photovoltaic panel and all the heat was coming from light being concentrated in from this mirror. The light would actually come right in through this window on the front; this is just a steel port, a transparent ceramic window and it’s like a greenhouse, but the temperature would be several hundred degrees. All of the light from all of these mirrors is focused through this one single window. This particular roaster we’ve had up to seven or eight hundred degrees; at that temperature it just vaporizes the coffee. You get charcoal and smoke.”

“This roaster (the 2nd generation Helios 2) is just simple rows of mirrors and the effect is very much the same. You can still focus the light just as effectively. We use glass mirrors on this one because they stay reflective for a lot longer. Plastic gets scratched and if you try to clean it, it ends up becoming less and less reflective. As long as you don’t break the glass they reflect as bright as when you first got them.”

How expensive is this to build?

“Helios 1 cost probably four or five hundred dollars; we just scavenged. Helios 2 was on the order of a couple thousand dollars: the hardware, the mirrors and having a little bit of it fabricated.”

Does it pay for itself?

“The system we’re using in Pueblo is paying for itself. We have another one similar to this (the 3rd generation Helios 3) and it’s paying for itself in the sense that all of the coffee for our business is being made with it and it’s our primary coffee roaster. We’re finally making money with it.”

Is all your coffee being roasted by the sun?

It is 100% solar roasted. That is one reason we had to move there (Pueblo, Colorado). During the summer months we had no problem as it is very sunny here, but as soon as it starts to rain, in September, October, November, we are up a creek. The rule is if you can’t see your shadow you can’t roast coffee. If it’s hazy it starts to cut into it a little bit and if it’s very bright, but overcast, you have no ability to roast.”

So this type of thing can’t be reproduced in any location?

“You’d have to do it where it’s sunny most of the time or if you’re just doing it for a hobby you could just do it during the summer. It won’t work everywhere.”

(David walks over to where pieces of a metal structure are laying on the ground).

“The new coffee roaster, the new system of reflectors, is built out of these giant panels. This is a tower that sits in the middle of our new coffee roaster. It’s a giant solar tower that’s bringing in more light so this actually holds the receiver window that the concentrator is reflecting light onto.”

What’s the difference between this and the older versions? What are you trying to do this time?

“Instead of the coffee roaster being out on the end of an arm with this version you’d have to stand outside on a ladder and the coffee roaster is elevated up into the focus. With the new one we’re focusing all of the light onto a receiver that gets very hot and we draw in air through the receiver and we’ll be bringing super heated air down and into the coffee roaster. That will let us roast coffee without having to be up in the middle of the focus.”

“Like before we’re using mirrors that concentrate the light to a single spot. Whereas our old design concentrates about 6 kilowatts of power, about 6 sq meters, our new one concentrates about 24 sq meters, about 24 kilowatts. The old reflector is about 8 feet across the new one is about 35 feet across. With the first roaster we were able to roast about one pound, with the second roaster we could roast about 2 1/2 or 3 lbs, the third roaster that we’re using in Pueblo now we can roast about 5 lbs, and the new one will roast about 30 lbs.”

(David walks over to a concentrator that looks like half of a skateboard halfpipe).

“So this is a good example of a trough type concentrator. If you look at the edge it has a big parabolic curve; it’s like a parabola or a cross section through a satellite dish, but it’s very long so the focus is actually right here. If you put a black pipe through the focus you can boil water with this; it takes about 2 to 3 minutes to get it up to the boiling temperature.”

Where did this material come from?

“The mirrors I bought as 1 ft sq. mirror tiles, built a little frame, and used a glass cutter. I was able to just cut strips; it took about 3 or 4 hours to cut the box of mirrors into strips. The rest is made from scratch; it is lumber; probably about $300 in lumber. I’d say the total cost for this was $350. It took a good week to put it together. (He points to the rotating device at the base). This is a piece of an old furniture rack and it’s smooth painted metal so it slides off the carpet. The overall effect is that you can roll it and it’s really pretty smooth. There are no bearings; it’s really cheap; just bolted together.”

“This was an experiment that I was working on, trying to determine one type of architecture for the design for the concentrator. One possible design I thought about was to use a trough type design instead of the big dish. I would have a long trough that would concentrate the light onto a pipe. I built a few small troughs and then I decided I’d build one-quarter sized scale system just to experiment to see if it would work. The result was that it doesn’t get hot enough. The experiment showed me it was not the right thing to go with. We have a giant solar BBQ now.”

This is probably a little cumbersome for the average family…

“Concentrators really aren’t the thing to have at your home, I don’t think. In most cases, for home solar applications, people want to have electricity and the least maintenance way to do it is with solar panels with, photovoltaics, or with flat panels and making hot water. As soon as you have a concentrator you have to have something that follows the sun. This system is a little bit more expensive and a little bit larger than most people have reason to deal with.”

“It’s not reasonable to think that everyone will put a huge device on top of their house. They shouldn’t have to; cities should have them and communities should have them, especially in sunny areas. States and towns should have large scale solar facilities.”

Do you think your coffee is different because it’s been sun roasted?

“I think the coffee is better. Some of it is the fact that we’re not using any combustible materials to do the heating. We’re just putting heat in and some of it again is just the preferences and knack of the coffee roaster, my brother. Whatever the combination, it seems to be a good one.”