Steam turbines have row after row of blades, each successive blade wheel larger and lower pressure than the last. Exit temperatures from modern turbogenerators are around 120° C, where the steam is about to turn into water, and pressure is usually below atmospheric. Almost all the energy which thermodynamics permits to be extracted has been extracted.
I probably should have phrased it better - I was picturing [Furnace] | [Thermophotovoltaics] | [Boiler], so the PV part gets first crack at the light/IR from the furnace and then the boiler cools the PV gear as it boils the water. This is also how it's suggested to be used in a paper linked elsewhere in the discussion above: https://www.sciencedirect.com/science/article/pii/S030626191...
If you make it harder to cool the turbine by putting those neat the radiator, efficiency will go down; if you make it harder to keep the heater hot, by putting those near the fire, efficiency will go down.
As animats already said, turbine efficiency on power plants is very near the theoretical optimum. The only way left to improve it is by increasing the temperature.
The furnace burns at over 900C but the superheated steam (which is your ‘hot’ temperature for your heat engine) is only at ~400C, you might lose some power output but you won’t be losing efficiency.
How about one of the authors talking about a credit-card sized device supplying 100 watts to quad-coptors using liquid fuel and staying airborn for 16+ hours. (https://youtu.be/lDxJsa8miNQ?t=3202)
True, though it is combined (in the article) with a maximum power consumption: "those that require as little as 100 watts, [such as] a lightweight unmanned aerial vehicle"
The original paper says "unmanned vehicles" (no aerial) and has a reference [0] to "unmanned undersea vehicles".
The article says "a wide range of applications, from those that require as little as 100 watts, [such as] a lightweight unmanned aerial vehicle, to 100 megawatts"
I read the 100 watts as referring to lightweight unmanned aerial vehicle.
100 megawatts is followed by "[providing] electricity for 36,000 homes."
The original paper does not mention lightweight aerial vehicles as an application, but a drone is mentioned in the title (and shown in the top image) of the article.
Applications mentioned in the paper are:
hybrid cars, unmanned vehicles, deep-space probes, energy storage, enabling efficient cogeneration systems for heat and electricity.
(A less-misleading title might even be: "New thermophotovoltaic engine for deep-space probes")
Also, the reported 29.1% efficiency have been reached at a temperature of 1,207 °C.