i would suggest this:

/castsequence reset=15/target [mod]renew, power word: shield, renew, renew, renew, renew, renew, renew, renew

(you seriously dont need that many renews, since renew doesnt stack on a single target)
this will reset the macro after 15 seconds or when you switch targets.
this SHOULD let you cast renew when you press any mod button with it (shift, alt or ctrl), just in case you want to switch targets and not cast pw:s but just cast renew only on the new target.

see if this works for you. cant really test it out now since im at work.