I have open source and open hardware sensor that would give you a working starting point: it is internet connected and transmits its temperature, humidity, and battery voltage every two minutes an will last for 3-5 years on 2xAA batteries. It is based on the M12 6LoWPAN module.
I'll try my best to grab touch on all of your questions:
Regarding band tradeoff:
433MHz, 915MHz, 2.4GHz
Range vs. antenna size is the clear tradeoff here. Free-space path loss is a function of wavelength so lower frequencies travel much farther for the same attenuation. BUT, in order to capitalize on this you'll also need a suitable antenna which also scales with wavelength. The 2.4Ghz antenna on the M12 takes about 2 sq. cm of PCB area.
A second factor is licensing. 2.4GHz can have unlicensed stations worldwide. 915MHz is only unlicensed in US (it's a GSM band everywhere else). I'm not sure the restrictions on 433MHz.
Data rate is also effected by frequency choice according to Shannon–Hartley theorem; you can cram more data into a higher frequency band. This isn't always used for more final data rate though. 802.15.4, for instance, has 4 bits of redundancy for every real bit seen at the data layer. The 32 symbols are pseudo-orthogonal so you have to corrupt several low level bits to cause an error. This allows 802.15.4 to operate under the noise floor (research suggests at -5dB SNR) and makes it relatively robust to interference.
Now on to the next hard topic,
low-power radio operation:
Compared to household battery sources (e.g. AA alkalines), even the "low-power" SoCs such as the mc13224v aren't very low power. The transmitters are around 30mA at 2-3.5V and the receivers are 25mA or so. Without turning the radio off and putting the CPU to sleep, this load will drain 2 AAs in a few days. The high power consumption of the receiver is often surprising to people and probably the biggest pain in developing low power radio systems. The implication is that to run for years, you can almost never transmit or listen.
The goal to get "year long" operation from 2xAA alkalines is to get the average current of the system to be < 50uA. Doing so puts you at years and up against the secondary effects from the batteries such as self-discharge and the 7 year self life for household batteries.
The best way to get under <50uA average is if your transceiver doesn't need to receive. If this is true, then you can "chirp" the data as quickly as possible and put the system into a low power mode (say approx 10uA) for most of the time. The TH12, for instance, transmits for about 10ms, but there is other overhead in the system regarding processing time and setup times for the sensor involved. The details can be worked out with a current probe and spreadsheet:
From that type of analysis you can work out what the run-life is going to be (assuming you have an accurate discharge curve for your battery).
If you do need to receive data on the low power side (e.g. to make a sleepy router in a mesh network) then the current state-of-the-art focuses on time division techniques. Some tightly synchronize the network, such as 802.15.4 beacons, and others use a "loose" system such as ContikiMAC (which can be easier to implement esp. if your hardware doesn't have a stable timebase).
Regardless, my experience shows that these methods are around 400uA average which puts you in the "months to maybe a year" run-time with 2xAAs.
Collisions:
My advice: don't worry about them for now. In other words do "aloha" (your option #1) where if you have data send it. If it collides then maybe resend it. (this depends on you goals). If you don't need to ensure that every sample is received then just try once and go to sleep right away.
You will find that the power consumption problem is so hard that the only solution will be a network that isn't transmitting much at all. If you just try, it will probably get through. If it doesn't, you can always try again later.
If you do need to make sure every datagram gets through then you will have to do some kind of ACK scheme. In the 6LoWPAN world, you can use TCP which will keep retrying until your battery is dead. There is also CoAP which uses UDP and has a retry mechanism (but doesn't promise delivery). But every choice here will impact run-time. If you are operating for years, the impact will be in months.
Your option #2 is built into 802.15.4 hardware as CCA. The idea is that receiver turns on for 8 symbols and returns true or false. Then you can make a decision about what to do next. You can play with these schemes all day/week. But every time you do something like this you shave more weeks off the run-time. That's why I suggest to start simple for now. It will work quite well if you are trying for long run-times.
I think you'd be hard pressed to get the required 150mW or so, although it isn't impossible.
Looking into piezo-based energy harvesting systems, one product claims about 7mW constant power. I would assume this scales reasonable well though. This is using a 3x7cm bendable piezo element and some electronics to store and convert the energy. There's a lot of info on piezo.com.
Using the device above you'd need 20-25 of these elements. As they flex quite a bit and obviously should not touch each other, you end up needing quite a large box.
If, however, you are able to design the radio so it only transmits at a low duty-cycle and goes into some deep-sleep power saving state if it doesn't transmit, this seems feasible.
Although he isn't using XBees (but RFM12B radios), have a look at the jeelabs.net website - there's some examples on low-power use of microcontroller driven radios.
With the added detail that the frequency of the vibration will be between 80-260Hz, have a look at this device. The specs state it can generate 7mW at 50Hz - they're not cheap though at $600 for a kit. And you'll be needing something in the order of 25 of them.
Two bonus links: Linear has done some research on this (you've probably seen this already), using this sensor.
Quote:
energy
harvesting can produce about 1mW–10mW,
where the active sensor-transmitter
combination may need 100mW–250mW.
Best Answer
Here is my guess regarding power harvesting: If you would make the power harvesting antenna reasonably large and have it always right next to a WiFi accesspoint or next to any other transmitter and these are operating 24/7 then yes, it can probably be done.
But for any practical application that you could place anywhere so not near to anything transmitting radio waves all the time: NO, there will not be enough energy available to do anything usefull.
You, like many others, are grossly overestimating how much energy you can "harvest" in a practical scenario. You also may want to have a look at this video.