Coal plant conversion to solar

How it works today
Simply, a coal plant heats up a tank of water until it turns into steam and that pressurized steam is used to turn a turbine which turns a generator to generate electricity. Currently coal power plants supply about 50% of the electricity in the US.

How it can work tomorrow
Instead of heating the water tank with coal fired burners, heat it with concentrated solar (simple mirrors are the cheapest). This is most efficiently done on plants that are being decomissioned anyway. Conversion of currently operational plants can be costly. The excess heat can also be used to generate a smaller amount of power by using a Stirling engine generator. On days where the sun is not very strong, the Stirling engine should still provide some power (all it needs is a difference in temperature, the larger the better). It is possible to use geothermal for the Stirling engine

The downside
Cost, there will be a cost associated in installing the mirrors and preparing the system to be heated by solar. Not all locations will be appropriate for this application, there may not be enough sunshine available. The plant will be able to supply power when the sun is hot enough to heat up the water.