The replacement cost occurs when the component needs replacement.  Your PV array and your wind turbine both have a lifetime of 20 years which is the same as the project lifetime, so neither requires replacement.  For that reason, HOMER won't use the replacement cost of those components so you can leave them at zero.

To see how HOMER uses the replacement cost in its economic calculations, please look up "annualized replacement cost" in the index of the Help system.  In short, the replacement cost occurs each time the component needs replacmenet, and it also affects the salvage value of the component at the end of the project lifetime.  If the component lifetime is equal to the project lifetime, the replacement cost has no effect.

By default, HOMER makes the replacement cost equal to the capital cost, and in the publication you're referring to it's likely that the modeler used that assumption.