What are some alternative power sources for wireless sensors?

I'm looking to power several small, Wi-Fi enabled sensors, in a domestic home or office environment. As such I'm interested in keeping them powered as long as possible between charges.

Obviously an expensive Li-xx battery would be simple solution, but I've also been looking for more 'inspired' alternatives, such as Micropelt thermogenerators.

What other alternatives are there that would provide a decent amount of power, in a small size?

Alternative wireless networking ideas are welcome, but I'm keen to use WiFi - most homes have a WiFi network and enabled computer - I'd like to keep the extra equipment needed to an absolute minimum.

[There is a related question, about energy storage. My question is about generation and my needs are fairly specific (size, high power etc.]

Answer

You should specify what sort of environment you're talking about. Most of the methods I've heard of also use batteries to store the energy, but you could also use an array of supercapacitors. However, that's getting expensive as well.

Ideas I've seen used successfully to some degree:

Solar is the first one, but it does require light. Any light will do of course, even fluorescents. Will it keep your circuit powered for a significant amount of time? Depends on where it's put.

Vibration/Piezoelectric generation has been used successfully to power sensors placed under busy stairs. The key is busy stairs - imagine the New York subway or the main stairs at a university between classes.

EM capturing - If there's a lot of ambient EM then you can put out an antenna, rectify the results and regulate it - boom, power. However, it's usually illegal to do this with significant sources. For example, you can power a light bulb if you're with a quarter mile of a large radio antenna, but harvesting that power is illegal in most countries. It WILL be noticed too.

As Endolith said, you'll probably want a less power-hungry module. Consider Zigbee instead of WiFi - it was designed to be used in low-power devices that transmit for short bursts then stop.

[EDIT] Ok, office/home/domestic is the application. If you control it significantly, maybe you can alter it. Say by adding inductive chargers? I don't think there's that many source of power in a standard office other than solar.