Renewable project developers want long term power purchase agreements (PPAs) so they have predictable revenue for projects they plan to develop. These are agreements that go something like "for the next 15 years, we agree to buy up to X megawatt hours per year from your project for Y dollars per megawatt hour." There are other ways to sell output from renewables (like bidding output directly into the electricity spot market) but developers overwhelmingly prefer PPAs. Google has signed a bunch of PPAs:
https://cleantechnica.com/2016/01/29/google-signs-225-mw-win...Once the developer has signed PPAs they can finance the up-front construction cost. PPAs enable the construction and operation of new renewable energy projects. Of course the output of large scale renewable facilities is mixed with other sources of electricity in a transmission system; it doesn't directly replace coal. (But this is true of any electricity project in a competitive market: when a company builds a new natural gas plant, they don't detach the transmission wires from a competitor's older coal plant and attach them to the new gas turbine.)
Instead it's pricing effects that indirectly decrease use of dirtier fossils when a grid adds renewable capacity. Solar and wind plants have very low variable costs; they'll supply every MWh they can to the grid, and get paid for every one of them via the previously signed PPAs. But fossil generators have significant variable costs for fuel and for labor[1], so when renewables are producing high output there may not be enough demand to cover the marginal costs of production. Fossil plants reduce or stop production when that happens, so they burn less fuel and produce less emissions. Fossil plants with high enough marginal costs may shut down for extended periods, like some plants that are only used for e.g. meeting peak demand during the winter heating season. Costs rise over time as aging equipment needs additional maintenance or full replacement. Fossil plants with high enough costs eventually can't turn a profit even serving the peak-demand-season niche and are completely retired and scrapped. Even if an old coal plant is replaced by a new coal plant, it's still an environmental win to get the old ones shut down, because (even under the new president) a coal plant that starts construction in 2017 won't get the weaker grandfathered pollution standards of one constructed in 1957, and the newer plant will produce more electricity per ton of fuel due to technical advances.
[1] At least coal plants have significant labor costs. Modern CCGT natural gas plants are quite labor-thrifty too.