One of the useful attributes of electronic paper (such as E-Ink) is that it doesn't take any power to retain an image, it only takes power to change the image. This is good for long-lasting E-readers, and digital signs are one of the other key applications of electronic paper, though today they are sold with a focus on the retail market.

Earlier, I wrote about concepts for a fourth screen which is an always-on wall computer that has effectively no user interface -- its purpose is to show you stuff that is probably of interest to you based on time of day and who is looking at the screen. That proposal requires that the display be located where there is power, but there are many locations where wiring in permanent power is not a readily available option.

The typical e-book reader has all the hardware needed to act as a very low-power digital wall display. Such a display would have electronic paper and wifi. It would only wake up very rarely to briefly check, over the wifi (or better still bluetooth) if there is new data to display, in which case it would download it and display it. During these updates, it might also check to see if there is a new updating schedule.

You can do better than wifi, which usually requires a process of associating with an access point, getting an IP address, and then making queries. Bluetooth can connect with lower power. Even better would be a chip which is able to listen constantly at very low power for a special radio pulse ("wake on pulse") from a powered transmitter, and then power on the rest of the system for data transfer. The panel could be put anywhere, and then a pulse generator would be put somewhere nearby that has power and is close enough to wake up the panel. (It might be something that plugs into a wall outlet and even does networking over the power lines.) This would allow the valuable ability to push information to the panel.

The panel's battery would of course die in time, so there would need to be a battery swap ability or if need be a means to charge with a temporary extension cord, a battery-powered charger or taking the panel off the wall.

An immediate market for these would be the doors of meeting rooms, so that they can show the schedule for the meeting room. Many hotels and convention centers have screens to do this now, but due to the need for power and other integration, these tend to be quite expensive, while ebook readers are now in the $100 range.

But they would also be useful around the home for 4th screen applications, displaying useful info. They could also be put near fridges or stoves to display recipes and family information. Obviously if you can put in a powered LCD display, that's going to be able to do more, but without the power constraint more people might use it. They do need to be lit by external light, of course, but also are visible in bright sun in a way that lcds are not. And a product like this might well start eating into the retail digital signage market -- anybody know what the price points are these days in that market?