Wireless Sensors: Is Energy-Harvesting Possible?

We'll save ourselves and our customers significant money by using wireless sensors in places to connect multiple machines across varying distances, many in the 1000-ft range.

First issue: Update rates will vary from 10/sec to 1/sec with some others transmitting on status-change only. So we have options for powering the sensors. At these rates, can we use energy-harvesting methods? What's the general state of battery life for these conditions? There even are a few spots where PV is an option.

Second issue: We hear that the level of encryption has a big effect on battery life, which no one seems to talk about. Any practical advice for these issues?