The Stirling engine is a piston engine that can operate with a variety of external heat sources (i.e., “external combustion”, when a fuel is burned to supply the heat). A contained gas, typically helium, is cyclically heated to high temperatures and pressures to provide force on a piston, which then drives an electric generator. The difference in temperature between the heat source and a heat sink (typically the atmosphere) and the effectiveness of the transfer of heat into and out of the working gas is what determines its performance. Stirling engines are operating today with fuels as diverse as natural gas, fuel oil, and biomass (e.g., wood chips) and also with concentrated solar energy. Stirling engines typically have low maintenance and high reliability, and when paired with clean fuels can be an environmentally friendly way to generate power.
There have been many attempts over many decades to develop a commercially viable stirling engine, and recently there has been renewed interest and progress. (Try putting “stirling engine” in to a search engine like google, and stand back.)
Here are a few of the current players:
– STM Power Inc., Ann Arbor MI, has attracted industry and investor interest with their four-cylinder “swash-plate” design. http://www.stmpower.com
– Stirling Energy Systems, Phoenix AZ, plans to use the Swedish Kockums engine in solar dish concentrator developed with Boeing (actually Northrup). Rumor has it the company may be shortlived, having not been able to raise funding.
– Whisper Tech is a recent entrant from New Zealand, focused on residential cogeneration (aka micro combined heat and power – MCHP).
– Sigma PCP developed in Norway (www.sigma-el.com) is now a wholly owned subsidiary of Ocean Power (http://www.powerco.com); also directed at MCHP.
Alternative Designs, Inc. (ADI) has developed an advanced “Dual Shell” Stirling engine which permits operation at higher temperatures and thus higher efficiencies. They believe that this “Dual Shell” system and other improvements will allow the company to achieve a fuel to electricity conversion efficiency as high as 50%. They estimate that their costs could drop to around $400/kw. The engine has multi-fuel capability and full power levels at high altitudes. The 25kw unit is small and compact– roughly 2 feet high and one foot in diameter.
In early 2001, ADI will complete a 25 kW prototype system and begin a performance validation program. Through early 2002, ADI will develop five additional prototypes and begin work to commercialize the product. ADI plans to sell complete power generation systems ranging from 25 to 100 kW beginning in late 2002.
ADI’s advanced Dual Shell system utilizes a host of patent pending and proprietary technologies that will significantly improve the efficiency of a Stirling engine while simplifying construction and reducing manufacturing costs. Principle among these are a dual pressure vessel design which allows the engine to operate at higher temperatures while still using relatively common materials, and a reduced cost heater head design which reduces the number of manufacturing steps by a factor of ten.
The are looking for equity investors and a strategic partner. I can supply additional details and a copy of the business plan.
Wayne Bliesner, President, 425-402-9632, firstname.lastname@example.org
ADI’s advanced Dual Shell system has the high efficiency and low system cost required for success in the power generation market. Its patented dual shell design enables the engine to operate at temperatures much higher than existing Stirling engines, increasing the relative efficiency by 20%. It also uses a specially designed regenerator that improves the relative efficiency another 5% by recycling the waste heat into the hot cycle of the working gas. On most Stirling engines the heater head component is responsible for 50% of the system cost — primarily due to the complicated series of welds required. On the advanced Dual Shell system a patented design reduces the number of welds from 280 to 30. ADI had also invented a proprietary process that allows all 30 welds to occur simultaneously in a single step. Tests of this process have been conducted with outstanding success. These and several other patented or proprietary design and manufacturing improvements will allow ADI to produce high quality, low cost engines.
ADI has used advanced modeling techniques and testing to reduce the development risk of the Dual Shell system. For instance, software developed by NASA to study Stirling engine performance has been used by both ADI and independently by NASA to validate ADI’s power and efficiency estimates. The results have instilled a high level of credibility in the advanced Dual Shell design. In addition, numerous prototypes of key components have been built to test and simplify manufacturing methods.
Reliability and flexibility have been designed in from the start. ADI projects that these engines can be run continuously for ten years with only four maintenance intervals. The basic system is expandable to allow easy development of higher power systems by simply connecting several units in series. (ADI has conceived of a way to allow each unit to be individually de-clutched from the power train so that, on the rare occasion that maintenance is required, the multi-unit generator set may remain safely and continuously on-line — operating at slightly higher capacity on the remaining units.)
ADI’s prototype engine drives a standard “off-the-shelf” electric generator at 1800 rpm to produce a minimum of 25 kW of continuous power at 480 Volts and 60 Hz.