Heatengines convert heat into work. They have an important role to play in the utilization of solar thermal power. The heat engine field has been dominated by steam turbines, which have been perfected in the last century, but which also have become very expensive. This page will mention a number of heat engine types that are less well known, but probably cheaper to make and use.
Tesla had a theory that said that the smoother the transition between different types of energy, the more efficient it will be.
More to follow. AMTEC Cells generate electric power by pushing evaporated alkali salts through a membrane at high temperatures.
From this : The literature suggests the potential use of a scroll compressor as a high efficiency expander (Wells, 2000).
Copeland® compressors have been used successfully as expanders with R-134A and R-245FA refrigerants as the
working fluid. Efficiencies over 70% were demonstrated when operated with pressure ratios between three and five
(Warner, Wayne – Copeland Corporation, Personal Conversation, 10 May 2004). Scroll expanders have also
been utilized in an organic Rankine micro combined heat and power system patented by Yates et al. in 2002
one manufaturer of scroll expanders
another good starting point
The Stirling engine comes in several guises, different types. It has been developed for combined heat and power generators and concetrated solar generators. This process has put this simple mechanism out of reach of the market, elimintaing some competition along the way. They should be as cheap as compressors, but they are not.
A Tesla turbine uses discs instead of bladed wheels and uses friction instead of momentum of pressurized steam to convert steam pressure into work.
'Researchers from the Energy research Centre of the Netherlands (ECN) have achieved a record efficiency in the conversion of heat to sound. In a so-called thermo-acoustic engine they have improved on the existing record of 41% to reach 48% of the maximum possible efficiency.'
'The traveling-wave engine converts 18 percent of the heat source energy into electricity. Since the only moving component in the device besides the helium gas itself is an ambient temperature piston, the device possesses the kind of high-reliability required of deep space probes.'
A Thermoacoustic engine leaches the energy of standing acoustic waves generated by a heat source. There have been inprovements in their efficiency. One is to allow the wave to resonate around a circular tube, another is to realize the air vibration can be turned into electricity using a low weight membrane
Here's a possible generator for thermoacoustic applications.
An expert with innovative ideas is Aster Thermoacoustics
A 5 hp stirling engine here
Metal hydride heat engine