Due to pressing political and environmental factors, the entire world is looking for alternative energy sources. For decades scientists have been researching everything from photovoltaic cells to solar troughs in an effort to find ways to use clean, renewable resources to generate power. Many of these breakthroughs can be expensive and difficult to maintain for such little power output compared with traditional power generation. Finally, an invention that is over 100 years old may provide a solution – the Stirling engine combined with solar power. (Shown on the right is a picture of the future solar plant)
A typical stirling engine consists of a reciprocating piston/cylinder arrangement in a closed regenerative cycle. Sealed inside the cylinder arrangement is a pressurized gaseous “working fluid” which follows ideal gas laws. The fluid flows back and forth between a hot heat exchanger, a regenerator (or temporary heat store) and a cold heat exchanger. Though usually hydrogen, the fluid can be helium, nitrogen, air, methane, or even ammonia. The fluid is heated by an external source, and the increased pressure in the heated cylinder pushes on the power piston. The movement of the piston turns a crankshaft which produces work. Materials with low coefficients of friction are used in the construction of the engine, and some designs eliminate sliding altogether by using diaphragms instead of pistons.
Theoretically, the stirling engine can perform to full Carnot Cycle efficiency, but there has been no success in practice. Depending on the configuration, efficiency typically hovers around 25-30%. It can run on a temperature difference as little as 7 ͦ C, with diminished power output, of course. The engine can also be supplied with mechanical power to reverse its cycle and be used as a heat pump.
The stirling cycle has four steps involved in its operation, illustrated in the animation below.
1. Heating: Energy is transferred from the heat source to the working fluid through the hot heat exchanger. This causes the pressure of the working fluid to increase within the fixed volume contained by the piston. The bulk of the fluid is contained in the hot cylinder at this time.
2. Expansion: At a certain point, the pressure within the cylinder becomes high enough to displace the power piston. The power piston turns the crankshaft which results in a power output. The working fluid will continue to expand until the pressure inside the cylinder reaches equilibrium with the pressure applied by the piston.
3. Cooling: On the cold side, energy is transferred from the working fluid to a heat sink through the cold heat exchanger. This causes a pressure decrease in the cold side. At this point, the working fluid is mostly inside the cold cylinder.
4. Compression: At a certain point, the pressure within the cylinder becomes low enough to displace the displacement piston, sending the cooled fluid back to the hot cylinder. The working fluid will continue to compress until the pressure inside the cylinder reaches equilibrium with the pressure applied by the piston.
The presence of the regenerator is really what distinguishes a Stirling engine from a simple hot air engine. There can be many different piston/cylinder configurations. Here are the three most common types:
Alpha: The alpha configuration consists of two connected cylinders (shown left) – one housing the power piston and one housing the displacement piston. The working fluid flows between the two cylinders during operation. This configuration yields a high power to volume ratio, but it can have some maintenance problems with the durability of the seals in the hot cylinder.
Beta: The beta configuration consists of two pistons moving within the same cylinder (shown right). This configuration is also known as displacement. The power piston is tightly fitted in the cylinder, while the displacement piston is loose
Gamma: The gamma configuration is basically the beta configuration with the power piston mounted in a separate cylinder. This configuration provides lower compression, but is mechanically simpler – so it is used more frequently in multi-cylinder engines.
There are also several other configurations that are less widely used. Sometimes a diaphragm is used instead of a piston in an effort to eliminate moving parts. A typical rotary engine qualifies as a Stirling engine. Even a thermoacoustic refrigerator is a Sterling engine – the device is different, but the gas still follows the Stirling cycle.
The Stirling engine has many benefits, most notably its ability to run on any available heat source. This fact alone makes it eco-friendly. It typically runs between 15% and 30% efficiency, and Stirling electricity generation is cost-competitive with traditional generation up to 100kW. It’s quieter, more reliable, and lower maintenance than an internal combustion engine – which is why it has been used to power submarines and some aircraft.
Unfortunately, the Stirling engine has a high capital cost as well as a high cost per unit power. Though it can operate on as little as a 7 ̊ C temperature difference, it needs a large difference to be efficient. It’s larger and heavier than a traditional internal combustion engine. The system cannot start instantly – it needs time to warm up and build momentum. By the same token, the power output is relatively steady and cannot be changed quickly.
Solar Stirling Engine
With the help of a large dish of mirrors, the solar Stirling engine can use the concentrated heat from the sun as fuel to produce work. This system, named as the SunCatcher (below, right), was developed by Stirling Energy Systems Inc. The SunCatcher is comprised of a concentrator and a power conversion unit (PCU: below, left) – the Stirling engine. The concentrator consists of 82 curved glass mirrors, each three feet by four feet in area, supported by a 38-foot diameter dish structure. Unlike some other power generation systems which require the combustion of fossil fuels, these mirrors allow the Stirling engine to utilize solar energy by concentrating it onto the hot heat exchanger.
The Stirling Energy Systems configuration consists of a 4-95 Stirling engine – that is, four cylinders, each containing 95cc of hydrogen gas (shown left). As the engine completes each cycle, it turns a small electric generator to produce power. Each system can generate up to 25 kW of electricity. Since the Stirling engine is the entire operation of the SunCatcher, increasing the efficiency of the engine will significantly benefit the overall system performance. The highest SunCatcher conversion efficiency ever recorded is 31.25 percent.
The SunCatcher is a clean system; the only fuel used is the sun and it does not produce any byproducts during electricity generation. Once operational, the only other resource it uses is a small amount of water to periodically clean the mirrors - 4.4 gallons per MWh of energy produced, which is much less than traditional power generation usage (which can be 250 gallons power MWh). Since the Stirling engine does not use internal combustion, it is a very quiet system, emitting less than 66 dB at full load. The SunCatcher is a standalone system; the whole system will not be affected much if there is a problem with one dish. Contrast this with a parabolic trough plant which focuses all energy on a central turbine – when the turbine is down for maintenance, power production halts. Moreover, the SunCatcher produces close to maximum output even when the sun is obscured or low in the sky. While the highest efficiency recorded is 31.25 percent, the SunCatcher’s full-year, sunrise-to-sunset efficiency is still a respectable 24 to 25 percent, roughly double the parabolic trough system’s efficiency. Although the Sun Catcher system has many advantages, the challenge remaining is to turn the prototype into a low-cost, mass-producible design.
There are six SunCatcher systems planted in the New Mexico desert near Albuquerque, but this is only a testing site of the Solar Thermal Test Facility at Sandia National Laboratories (shown right). Stirling Energy Systems, Inc has just signed two large contracts to provide power for Southern California. The future power plant will be located in the San Diego area, and will include 12,000 dishes with a 300MW transmission capacity. The remaining 24,000 dishes will be built only if San Diego Gas & Electric is able to complete a proposed 150-mile transmission line between the plant and the city.
• How Stirling Engines Work, How Stuff Works, http://auto.howstuffworks.com/stirling-engine.htm
• Sterling Engine Society, USA http://www.sesusa.org/
• Stirling Engine, Wikipedia Encyclopedia, 2008 http://en.wikipedia.org/wiki/Stirling_engine
• Solar Thermal Power May Make Sun-Powered Grid a Reality Popular mechanics magazine, nov 2008 http://www.popularmechanics.com/science/research/4288743.html?page=1
• Sun Catcher description, Stirling Energy Systems http://www.stirlingenergy.com/technology/suncatcher.asp
• Technology, Stirling Energy Systems http://www.stirlingenergy.com/technology/default.asp