Applications of the Stirling engine

[3] There are several design configurations for Stirling engines that can be built (many of which require rotary or sliding seals) which can introduce difficult tradeoffs between frictional losses and refrigerant leakage.

For example, a free-piston Stirling cooler (FPSC) can convert an electrical energy input into a practical heat pump effect, used for high-efficiency portable refrigerators and freezers.

[3][4] It is often claimed that the Stirling engine has too low a power/weight ratio, too high a cost, and too long a starting time for automotive applications.

Automobiles exclusively powered by Stirling engines were developed in test projects by NASA, as well as earlier projects by the Ford Motor Company using engines provided by Philips,[6] and by American Motors Corporation (AMC) with several cars equipped with units from Sweden's United Stirling built under a license from Philips.

[12] The process of auto manufacturers converting their existing facilities and tooling for the mass production of a completely new design and type of powerplant was also questioned.

In a petro-hydraulic hybrid the Stirling would perform a similar function as in a petro-electric series-hybrid turning a pump charging a hydraulic buffer tank.

In November 2007, a prototype hybrid car using solid biofuel and a Stirling engine was announced by the Precer project in Sweden.

[18] The engine was attached to the front of a modified Super Malibu radio control glider with a gross takeoff weight of 1 kg.

The best-published test flight lasted 6 minutes and exhibited "barely enough power to make the occasional gentle turn and maintain altitude".

General Motors has undertaken work on advanced Stirling cycle engines which include thermal storage for underwater applications.

United Stirling, in Malmö, Sweden, are developing an experimental four–cylinder engine using hydrogen peroxide as an oxidant in underwater power systems.

[23][24] WhisperGen, a New Zealand firm with offices in Christchurch, has developed an "AC Micro Combined Heat and Power" Stirling cycle engine.

These microCHP units are gas-fired central heating boilers that sell unused power back into the electricity grid.

These systems, on an 8,000 acre (19 km2) solar farm will use mirrors to direct and concentrate sunlight onto the engines which will in turn drive generators.

"[26] The SunCatcher is described as "a large, tracking, concentrating solar power (CSP) dish collector that generates 25 kilowatts (kW) of electricity in full sun.

[29] During the Space Shuttle program, NASA successfully lofted a Stirling cycle cooler in a form "similar in size and shape to the small domestic units often used in college dormitories" for use in the Life Science Laboratory.

[citation needed] Any Stirling engine will also work in reverse as a heat pump: when mechanical energy is applied to the shaft, a temperature difference appears between the reservoirs.

At typical refrigeration temperatures, Stirling coolers are generally not economically competitive with the less expensive mainstream Rankine cooling systems, because they are less energy-efficient.

Stirling cryocoolers are able to "lift" heat down to −200 °C (73 K), which is sufficient to liquefy air (specifically the primary constituent gases oxygen, nitrogen and argon).

For this application, Stirling cryocoolers are the highest-performance technology available, due to their ability to lift heat efficiently at very low temperatures.

When the mechanical work for the heat pump is provided by a second Stirling engine, then the overall system is called a "heat-driven heatpump".

Stirling systems have seen limited commercial use; however, use is expected to increase along with market demand for energy conservation, and adoption will likely be accelerated by technological refinements.

The free-piston Stirling cooler (FPSC) is a completely sealed heat transfer system that has only two moving parts (a piston and a displacer), and which can use helium as the working fluid.

[33] Claimed advantages for the system include improved efficiency and cooling capacity, lighter weight, smaller size and better controllability.

[37] Other suppliers of FPSC technology include the Twinbird Corporation of Japan[34] and Global Cooling of the Netherlands, which (like Sunpower) has a research center in Athens, Ohio.

[38] For several years starting around 2004, the Coleman Company sold a version of the Twinbird "SC-C925 Portable Freezer Cooler 25L" under its own brand name,[39][40] but it has since discontinued offering the product.

[42] Other variants offered by Twinbird include a portable deep freezer (to −80 °C), collapsible coolers, and a model for transporting blood and vaccine.

However, larger (typically 1 m square) low temperature engines have been built for pumping water using direct sunlight with minimal or no magnification.

An agreement has been signed with Mondragon Corporación Cooperativa, a Spanish firm, to produce WhisperGen's microCHP (Combined Heat and Power) and make them available for the domestic market in Europe.

However the preliminary results of an Energy Saving Trust review of the performance of the WhisperGen microCHP units suggested that their advantages were marginal at best in most homes.