It's all about scale
Peltier devices are notoriously inefficient - only about 5%.
However, steam requires an army of skilled engineers present at all times the boiler is running (and hours before and after). It doesn't matter if you're making 500 milliwatts or 500 megawatts, you still need the army of engineers to avoid a deadly BLEVE (boiling liquid expanding vapor explosion). You can get 30-40% efficiency from a well-optimized steam boiler/turbine.
Note that 343K is much too cold for a steam plant to operate efficiently. You might be able to make it work with lower boiling point liquids like pentane if you can access an ultimate heat sink that is sufficiently cool. However, most of those fluids are flammable.
Even operating in efficient temperatures, steam is economically impracticable (depending on the cost of labor in your country) below a break point of 10 megawatts to 200 megawatts. At the lower ends of that range, you are much better off using gas turbines or plain diesels, which require a much smaller engineering staff, because they can't explode in the violent manner of a BLEVE. These run about 30-40% thermally efficient, net.
If you have a gas turbine installation large enough, you can use the rather hot exhaust to boil steam and run a steam plant alongside. This is called a combined-cycle plant. In this case, the boiler management staff are managing a number of small boilers (at each turbine exhaust) feeding into one large steam turbine. These combined-cycle plants have efficiency of 50% or even as high as 55-60%.