The difference between actual energy recovered and potential energy available, i.e., the 70% to 40% loss in efficiency, is a result of several factors, including inadequate or incomplete combustion of the fuel, generation of wasted heat, frictional losses in the mechanisms used to transform the chemical energy released in combustion to physical energy in the output shaft, exhausting of the combusted mixture before complete recovery of the energy thereof, etc.
Each of these factors adds to yield a relatively inefficient internal combustion engine.
However, gas turbines as a primary engine and without a method of secondary heat recovery are less efficient than diesel cycle engines, which is currently, on a stand alone basis (i.e., no secondary heat recovery based power generation) the most efficient engines commercially available.
Further, engines operating on the Stirling cycle would theoretically be more efficient, but have never gained commercial acceptance.
The relatively efficient diesel engine using commercial fuels has an exhaust temperature insufficient for efficient steam turbine power generation therewith, whereas the gas turbine has high enough combustion temperature, and exhaust temperature, to allow sufficient heat recovery for commercial uses.
However, the engine also released, as exhaust, unacceptably rich emissions of greenhouse gasses as nitrogen oxides, approximately double that of a non-oxygen enriched diesel cycle engine, and also was unable to be effectively controlled.
As a result, this concept has not been further pursued.
An ongoing issue with the use of fossil fuels or other hydrocarbons in conjunction with internal combustion engines is the generation of pollutants, such as NOx or COX compounds.
A portion of these emissions, specifically the NOX compounds, are known to cause disruption of the ozone layer, and / or smog, as well as being generally unhealthy when inhaled.
CO is toxic, as is an additional emission gas, CHX, Likewise, CO2 has been implicated in global warming, and the emission of it may become limited in the future.
Thus, although the efficiency of the engine can be increased, the resulting pollution is unacceptable.
Further, these peak plants are often operated only during peak demand periods, and thus their cost, i.e., the investment in infrastructure, is not recoverable based upon continuous generation, but rather based on less that full utilization.
Although SEGS have proven to be capable of providing power during peak operation times, there are limits of competitiveness which affect their use for base line power generation needs.
As the plants cannot operate in non-daylight hours, the cost of building the solar power generation equipment must be justified based solely upon generation during these daylight hours.
However, because of the limitations on the use of fossil fuels and the requirement that the plant must be able to produce 100% of its rated output from solar alone to receive preferential supply status and certain tax and other benefits of being considered solar, the fossil fuel based generation is minimally used and sub-optimum power generation equipment is used.
For example, although it may be reasonable to combine gas turbine and solar generation, the cost effective solar plants available before this new technology are not able to produce for technical reasons the full rated power of the plant from solar alone.
Thus a current SEGS plant cannot operate highly efficient combined gas-steam cycle turbines and still be considered a solar plant in many if not all locales.
However for many reasons most of which can ultimately be related to lack of sufficient material technology at this stage this approach is too expensive, inefficient and unreliable to be developed in to a commercial product.