I don't think you should use plastic bottles due to their low melting point. You could use glass bottles instead. Your design uses bottles as boilers and collectors. The problem is, the boiler and collector are some of the simplest and cheapest parts already. What you need are cheap steam engines, generators and condensers. For a cheap generator, you might be able to use a used car alternator. For a cheap condenser, you could try a used car radiator. The steam engine could be the tough one.

I don't know if having the system reverse direction (one bottle to the other, then back) is as practical as having it run in a continuous cycle. I wish you could post a diagram of how your system works so I could better analyse it.

I agree with Dwane, a diagram would be helpful.

The idea of using a plastic (PET) pressure vessel in direct sun seems like a short-lived system. I think it would break down quickly and the pressure would rupture it.

I like the continuous cycle idea, and keeping the boiling heat inside a metal chamber could work well.

The design could be all sots of things, but the idea is to use steam & reflection instead of solar panels, which are expensive, short-lived, and a bio-hazard. Think small & cheap.