Environmental Impact:

In an exclusive solar mode, the system has no emissions. In mixed or non-solar mode the use of very low intensity external surface combustion methods maintains nitrous oxide emission levels below 2 parts per million, and since the system operates at over 40% thermal efficiency over a wide load range, CO₂  emissions per kilowatt of electrical output are a very small fraction of what equivalent conventional power plants would yield.

Prospective Availability Improvements

The same fundamental reasons that gas turbines brought new dimensions to aircraft availability apply to the application of dynamic machines to Solar Power Conversion Systems. Radial piston aircraft engines had overhaul lives of 1,000 to 1,600 hours. In contrast, jet propulsion engines routinely go 10,000 hours between overhauls.

3,000 rpm internal combustion engines driving stationary generators have overhaul lives of 10,000 hours, and slow-speed diesel generator engines can go as high as 15,000 to 16,000 hours.

Gas turbine generators weigh much less and offer significantly longer lives between overhauls Many stationary gas turbine installations have operated continuously for well over 75,000 hours without overhaul. In rugged applications  such as off road heavy equipment the turbo chargers have outlasted the diesel engines by several overhaul cycles.

Reliability Rationale:

In the piston engine era in-flight failures were so common as to require at least four engines for trans-ocean passenger flights. Current civil transport jets require only two engines for the same flights.

Stationary power plants are almost universally turbine-driven. Small generator sets will follow the same trend as micro turbines as represented by Capstone begin routinely to log 25,000 hours and more without unscheduled service interruptions.

Stirling Cycle Engines:

Solar PCS applications heavily favor externally heated engine cycles to permit the use of open receivers without quartz window losses. This brings the competition down to a choice between Stirling cycle reciprocating engines and closed loop Brayton cycle gas turbines. There is abundant evidence of excellent reliability being achieved by small Stirling cycle engines that use flexures instead of piston displacement in space applications. Few Stirling engine manufacturers publish durability data. There is mention in the literature of a V160 Stirling engine driving a 6.5 kilowatt generator at 1,800-rpm for which a life of 5,000 hours is claimed. It is generally acknowledged that the four-cylinder Stirling engines used so far in solar dish applications are prone to mechanical problems.

Gas Turbine Systems:

The same turbo machines to be used in the CEC Solar PCS have been used by Ballard Generating Systems in a 250 kW, stationary, fuel cell with a design overhaul life of over 9,000 hours (one year). These rugged turbochargers are designed such that only the bearings and seal need replacement. By utilizing components fabricated from inconel, titanium and stainless steel alloy these units have a design service life of over 10 years. Currently Ballard is working towards a goal of operating 15 complete field test systems without a shutdown for one year.

The first of the 15 Ballard systems has completed its field trials successfully after several years of uninterrupted operation. The turbo chargers were returned for teardown examination, as shown in the adjacent photograph. Both low- and high-pressure spools were found to be in excellent condition except for the normal buildup of varnish in the bearing seals, which would not have inhibited continued operation.

The CEC system has been demonstrated in laboratory tests using variable high pressure turbine backpressure to simulate the turbo generator load, and screens on the low pressure compressor inlet to verify sub-atmospheric performance.

Economic Viability

In a 1999 study for the DOE, Arthur D. Little arrived at the following projection of CPS economics based on the use of Stirling engines in various future time periods: