Distributed Generation: Reciprocating Engines, Microturbines, Fuel Cells, Stirling Engines, and Photovoltaics

The major small-scale DG options are summarized in Table 1.

Reciprocating engines (see Figure 1) drive the vast majority of on-site generation. They are mass-produced by many manufacturers around the world, cost less than other DG technologies, and have a fully developed sales, maintenance, and repair infrastructure. All of these factors, combined with market familiarity, decreasing exhaust emissions, extended service intervals, and long engine life, continue to make reciprocating engines the most commonly used DG technology.

Electrical conversion efficiencies for natural gas-fired reciprocating engines in the 5-kW range are about 24 percent, based on the lower heating value (LHV) of the fuel. For larger engines in the 250-kW and higher range, efficiency can exceed 33 percent LHV. If the thermal energy is used as well, overall efficiency can approach 80 percent.

Diesel-fueled reciprocating engines are even more common than natural gas-fueled engines, but most of the diesel units are used strictly for backup power during grid outages, due to diesel engines' higher emissions. Many air districts will only allow these units to run for a limited number of hours per year.

Dual-fuel engines are growing in popularity. These units use a small amount of diesel for start-up and then run on natural gas. Emissions are reduced nearly to the level of natural gas engines. These units can be operated on 100 percent diesel fuel at times when natural gas is not available. A number of engine manufacturers make dual-fuel units, and existing diesel gensets can also be retrofitted to dual-fuel at a reasonable cost.

Derived from jet engine technology, industrial gas turbines for DG use range in size from 50 kW to tens of megawatts. They are particularly suitable for large cogeneration applications where high-temperature steam is needed. Emissions are lower than those of reciprocating engines, and nitrogen oxide (NOx) emission-control technology is available to reduce emissions even further. Other advantages over reciprocating engines are that these gas turbines require less maintenance, are lighter, and take up less space. On the other hand, for units under 2 MW, electrical efficiency is lower than for reciprocating engines, and the gas turbines take longer to ramp up and shut down.

Microturbines became commercially available in 1998. Although they have a slightly higher first cost ($1,000/kW), they are currently the most cost-effective alternative to reciprocating engines for small-scale generation (see Figure 2). Microturbines operate on the same basic thermodynamic principle, the Brayton cycle, as their larger cousin, the conventional gas turbine. However they are much smaller than gas turbines, with output measured in the tens to hundreds of kilowatts, rather than megawatts.

Microturbines have far fewer moving parts than reciprocating engines, so they have the potential for longer lifetimes with lower maintenance requirements. They also offer much lower emissions than comparable reciprocating engines, and their waste heat can be used for heating and cooling applications to bring total efficiencies up to 80 percent or more.

Microturbines have several advantages for niche applications. They are good at handling low-quality gases, such as "sour gas" at oil and gas resource recovery sites, and biogas from landfills, wastewater treatment plants, and agricultural livestock operations. Also, their exhaust gas stream is clean enough and hot enough to be used directly in greenhouses (the CO2 boosts plant growth) or in industries with drying processes such as brick, grain, or chemical drying.

There are more than 2,100 fuel cells operating throughout the world, most of which are precommercial units. First costs are still high for fuel cells ($4,000/kW at a minimum), and only a limited number of fuel cell products are commercially available. Fuel cells use chemical reactions rather than combustion to produce both electricity and thermal energy. Because there is no combustion, harmful emissions are extremely low—the only byproduct of hydrogen fuel cell electricity generation is pure water and heat. Most fuel cells currently derive the hydrogen from another fuel, using a "reformer" that is either integrated inside the unit or placed right next to it. This does create pollutants, such as trace amounts of NOx, although the process is still cleaner than combustion. The reforming process also produces carbon dioxide, although again, less than would be produced by most other fossil-fueled DG technologies.

Noise from fuel cells is very low compared with other DG technologies, and it generally only comes from air blowers and water pumps in the cooling module.

Fuel cell systems can also use waste heat to boost thermal efficiency to 80 percent or higher.

There are several different types of fuel cells, named by the type of electrolyte they use—phosphoric acid, proton exchange membrane (PEM), solid oxide (SOFC), molten carbonate (MCFC), and alkaline.

Phosphoric acid. The first commercially available product, introduced in 1992, is the PC 25 phosphoric acid fuel cell from UTC Fuel Cells, a unit of United Technologies. More than 200 of these 200-kW units have been installed worldwide. Fuji Electric also has phosphoric acid fuel cell systems commercially available, sized at 100 kW. Phosphoric acid fuel cells have proven to be reliable; the downsides are that their efficiency is about 10 percentage points less than solid oxide or molten carbonate fuel cells, and their purchase price is not expected to come down a significant amount from the current $4,000/kW. UTC Fuel Cells has said it is phasing out production of its phosphoric acid fuel cells in favor of proton exchange membrane fuel cells.

Proton exchange membrane. More than a dozen companies are developing PEM fuel cells. Ballard has a commercial unit and Plug Power has near-commercial units. Most stationary PEM fuel cells will have capacities under 10 kW. The efficiency of PEMs is lower than that of solid oxide and molten carbonate fuel cells, but PEMs are able to ramp up quickly and match a changing load. PEMs are the type of fuel cells that most car manufacturers are investing in for future use in automobiles.

Solid oxide. SOFCs are being developed in sizes ranging from 5 kW to multi-megawatts. A number of companies are active in this area, including Siemens Westinghouse. SOFCs have very high operating temperatures and therefore produce higher-quality heat and are more efficient than PEM fuel cells and other DG technologies. A downside is that they can't ramp up or down as quickly as PEM fuel cells.

Molten carbonate. MCFCs are commercially available from FuelCell Energy, in sizes ranging from 250 kW to 3 MW. As with SOFCs, MCFCs have very high operating temperatures and therefore produce higher-quality heat and are more efficient than PEM fuel cells or other DG technologies. However, they can't ramp up or down as quickly.

Stirling engines (see Figure 3) offer the potential for low maintenance and low levels of emissions. In recent years, three manufacturers have brought product to market for the first time after many years of development work. In North America, STM Power offers 55-kW units that are able to run on a variety of fuels, including natural gas, biogas, and palm oil. And in Europe, Whisper Tech and Solo offer natural gas-fueled 1-kW and 10-kW units. Other companies are working on larger systems, including engines powered by solar concentrators.

The prices of Stirling engines vary considerably depending on the size of the engine and the application. STM Power's 55-kW product sells for $1,200/kW, while Whisper Tech's 1-kW unit#8212;able to supply all of a home's space heating and hot water needs, together with about one-third of its electricity#8212;sells for around $2,600 in the UK.

Photovoltaic (PV) systems, which generate electricity directly from sunlight, are still expensive ($4,000 to $10,000 per kW), but their use is growing by 20 to 30 percent annually thanks to their many benefits. They are quiet and dependable, have no moving parts, easily scale to any system size desired, offer built-in price protection against fuel-cost escalation, and are often covered by utility and government financial incentives. Such incentives are available from a number of public and private entities, including the Sacramento Municipal Utility District, the Los Angeles Department of Water and Power, California's investor-owned utilities, Western Solar Utility Network, the State of Illinois, and the New York State Energy Research and Development Authority.

There are two basic types of PV modules: crystalline silicon and thin film. Crystalline silicon modules are used in almost all commercial PV systems. In this design, silicon (which starts out as sand) is mixed with a small amount of a substance with a different number of electrons (such as boron or phosphorus). When light hits the PV material, electrons are dislodged. This movement of electrons creates an electric current. Crystalline silicon modules have reasonably high conversion efficiencies (typically 12 to 14 percent) and are made from readily available materials. Unfortunately, they are expensive to manufacture.

The newer thin-film PV technology works on the same general principles as crystalline silicon modules but has the advantage of generating electricity from a very thin film. This means the modules could be integrated into building materials (such as roofing tiles). One type of thin film, amorphous silicon, is already commonly used for solar-powered consumer products (such as watches and calculators). In general, thin-film PVs require less material to manufacture than do crystalline silicon PVs, and they will likely be easier to produce on a large scale.

PV modules (also called panels) typically have a peak power output of 50 to 300 watts. Modules can be assembled into arrays, which can vary from just two modules for a small residential system to hundreds of modules for a utility-scale system of 100 kW or more.

Consider the impact on power quality and reliability. The power quality and reliability markets for DG are already quite large: more than 14 gigawatts of reciprocating-engine generators sized between 10 kW and 2 MW are sold worldwide each year into these markets. DG was once only of interest to equipment manufacturers, their dealer networks, and specific third-party companies. Now regulated utilities, their unregulated subsidiaries, and independent companies are starting to use DG in service offerings that provide enhanced power reliability and power quality. Though most energy providers are still using reciprocating engines, some are beginning to offer emerging technologies such as fuel cells and microturbines. At the same time, end users are deploying increasingly sophisticated automated control systems and data storage technologies that need premium power to ensure the quality and integrity of their processes.

Match the engine to heating and cooling applications. Most of the DG technologies described here produce heat that can be recaptured for other uses, thereby greatly improving the economics of DG projects. Reciprocating engines, microturbines, fuel cells, and Stirling engines produce heat that can readily be used for water and space heating. Waste heat from solid oxide fuel cells is hot enough to serve as input to a gas turbine for the generation of electricity. In addition, it can be used in high-temperature industrial processes or to meet steam needs. The waste heat from these DG technologies could also be used to power an absorption chiller.

DG installers typically recommend that CHP systems be sized to appropriately meet a facility's thermal load rather than its electric load, although this depends on the exact loads and costs. Vendors also say that facilities that have a simultaneous electric and thermal demand for at least 4,000 hours per year are better candidates for CHP, although the economics for buildings with 2,000 hours demand per year can still sometimes work.

Consider new technologies in areas with strict requirements on air emissions. Although emissions from reciprocating engines are being reduced as technologies improve, other options produce fewer emissions. PVs are the cleanest of all, and fuel cells are also quite clean. Microturbines also offer lower emissions than comparably sized reciprocating engines.

Numerous companies are pouring research and development funds into DG technologies. In the coming years, look for refinements in reciprocating engines, an increasing number of sites using microturbines, and the commercialization of new products from the manufacturers of fuel cells and Stirling engines. Developers of PV systems are also working on new materials and techniques that would make cells less expensive and more efficient.