'Phone power from hot coffee' debate heats up
Remember that USB-charging coffee coaster I blogged about a few days back? It used differences in temperature to generate electricity, meaning that your hot coffee or chilled beverage could charge your phone. Well, a few friends have been talking about how efficient it might be. Comments ranged from "could it really work?" to "it just seems like a fast way to cool down your coffee". Then, an friend of a friend who is an actual engineer asked some other friends about it, and they decided that it couldn't charge a whole phone.
One engineer said that at 60 kilojoules, there is probably about twice the energy of a phone battery stored in the average mug of hot coffee. But heat transfer systems don't work at 100% efficiency - they lose energy from heat while transferring it into a usable form of energy. the coffee coaster isn't transferring the heat in the coffee to an absolute cold - there is still heat in the room, meaning that not all the heat from the coffee can be transferred into energy. About 20% of the energy from the coffee can be transferred into energy at room temperature. That knocks the available energy from the cup of coffee down to about 12 Kilojoules (half a phone battery).
It doesn't stop there. You need a generator to transfer that heat energy into a form of useable electricity. That would work at about 50% efficiency, says our intrepid engineer, leaving roughly 6Kj available.
The engineer also points out that the average user won't be making all of that energy available to the coaster. They'll be drinking a lot of the coffee themselves, which means that it can't dump all its energy in the device. Even though you'll be filling up your coffee more than once, the fluctuation in volume will still lower the available energy for the coaster.
Furthermore, he points out that that the drink doesn't lose all of its heat via the bottom. "In reality almost all of the heat loss is going to be via the air and evaporation of the drink. I would be surprised if we get 5% over the bottom."
Ouch. So this coffee coaster isn't looking like the answer to our power problems after all. I took these comments and put them to Tom Joseph, founder of Epiphany Solar Water Systems, which is the firm behind Epiphany Labs. The estimates that the engineer used were mostly on the money, he says, but with one exception.
"We are able to get quite a bit better than 50% efficiency from the electrical generator portion for two reasons: 1) We use a tiny linear generator (think speaker coil) to convert the motion into high frequency AC electricity, so it's pretty efficient; and 2) Since the motor is heat powered, whatever losses occur in the generator are sent back to the "hot" side of the motor, so we can keep most of the losses inside the box."
Nevertheless, Tom agrees that there is no way to charge an entire phone using the coaster. "Even when fast charging a phone from a wall outlet, it takes 60-90 minutes and if your coffee or beer lasts that long you have bigger problems anyway," he says. "What we're offering is a convenient way to charge your phone with any heat disparity."
You could charge a full phone using a more permanent source with a good, solid temperature differential. Taping it against a cold window in the winter would work (especially if you're unlucky enough to be charging your phone in Saskatchewan, Alberta, or Manitoba). You could even use a candle, he points out.
"The ability to charge from a drink is a short term convenience to maintain a charge or save you when your battery is almost dead," Tom continues, "but not the ideal way to get a full charge unless you're a dedicated drinker (which means we should do great in Ireland!)."
Basically, the onE Puck is a fun gadget that will get you out of a tight spot if your phone is on its last few electrons and needs a quick emergency charge. "But the real significance here is in the global implications of a high efficiency, mass produced, low cost Stirling engine," he says. "This has been attempted for decades, but without success, for a variety of reasons that we think we can finally overcome. Imagine having the ability to power your whole house, or a whole village in the third world, without a connection to a power grid."
Epiphany will use the proceeds of the onE Puck to boost stirling engine production for its solar water systems, and provide some power into the bargain.
"Once we can provide both water and power, a huge portion of the world's population moves from survival mode into development mode," he says. "About 35% (and growing toward 50%) of the world's population is too busy surviving to be able to contribute to society on a higher level. If humanity is going to save ourselves from the mess we seem to be creating, we're going to need all hands on deck to help us find solutions faster than we can screw things up."
So this is about more than juicing up your phone. It's an innovative promotional item designed to draw attention to a bigger project that could have wide-reaching consequences. And it's from a company that epitomises social entrepreneurship. I'll raise my cup of piping hot beverage in a toast to that.Danny Bradbury, MSN Tech & Gadgets