Distributed Generation Technologies

Fuel Cells | Photovoltaics | Microturbines |Internal Combustion Engines | Wind Turbines | Combined Heat & Power

Fuel Cells

A fuel cell is an electrochemical device that generates electricity by combining hydrogen from a hydrogen-rich fuel (methane, methanol, propane, or biomass) with oxygen (typically from the air) to produce electricity, heat, and water. All fuel cells consist of anode, cathode and electrolyte; much like a battery, except that the reactant fuel is continuously fed to the cell. Electrochemical oxidation and reduction reactions take place at the electrodes to produce electrical current. Each individual fuel cell produces less than one volt of potential, so cells must be stacked to obtain the desired voltage. Typical fuel cell capacity ranges from 2 kilowatt (kW) to 2 MW and have electrical efficiencies that range from 45 to 65 percent. With heat recovery, the efficiency can be as high as 85 percent.

Four types of fuel cells are receiving the most attention today. They are the

• phosphoric acid fuel cell (PAFC),

• molten carbonate fuel cell (MCFC),

• solid oxide fuel cell (SOFC), and

• proton exchange membrane fuel cell (PEMFC).  

PEMFC's have just entered the market. Plug Power and General Electric (GE) are marketing a 7 kW PEMFC for residential and commercial use. Ballard Power Systems of Vancouver, British Columbia, is working with the automobile industry on development of transportation applications for the PEMFC. Low operating temperature, 200^oF (80^oC), means that this type of fuel cell would not be appropriate for CHP, but the low operating temperature allows the PEMFC to be brought up to operating temperature rapidly. Technical issues facing PEMFC include electro catalyst poisoning by carbon monoxide (CO), water management, balance of plant costs, and cell life.

PAFC's have been commercially available since 1993. Today they are marketed by ONSI Corporation, a subsidiary of International Fuel Cell Corporation, which has sold some 250 units worldwide. PAFC's have an operating temperature of 400^oF (200^oC) and have been demonstrated at sizes ranging from 50 kW to 200 kW. They are 40-50 percent efficient.

MCFC's are high temperature fuel cells that operate at 1,200^oF (650^oC) and are 65 percent efficient. These offer the potential for internal methane reforming and CHP applications for commercial, light industrial, and distributed power applications. Sizes range from 250 kW to 2.5 MW. Issues that need to be addressed before wide spread commercialization occurs are higher power density, cell life, cost reduction (from $1,000/kW to $200-400/ kW), thermal management, and reliability. Commercial availability for MCFC is projected for 2001-2002.  

SOFC's are projected to be commercially available as early as 2003. They operate in the range of 1,800^oF (1,000^oC), depending on whether the geometry is tubular or planar. SOFC have potential for use in all the traditional power generating markets including CHP. Relative to the other fuel cell types, SOFC is particularly dependent on development of suitable low-cost materials and fabrication techniques. Electrical efficiency for SOFCs is 46 percent.

Hydrogen, the required fuel source for fuel cells, can be produced from water using electrolysis,with the necessary electricity generated using renewable energy. The National Aeronautic and Space Administration (NASA) is currently working on a “regenerative fuel cell” that would be a closed-loop form of power generation. In the regenerative fuel cell, water is separated into hydrogen and oxygen by a solar-powered electrolyzer and fed into the fuel cell to produce electricity and water. The water is then re-circulated to the electrolyzer to complete the cycle.

However, because this method is relatively expensive, most fuel cell systems use some form of hydrocarbon fuel as their hydrogen source. The use of a hydrocarbon fuel produces gaseous, liquid, or solid waste by-products. 

Some source compounds will have fewer and smaller amounts of by-products. The following is a list of hydrogen sources that rank from lowest to highest in by-products:

1. Water

2. Methane

3. Propane and natural gas

4. Gasoline

5. Fuel oil

6. Gasified coal

Fuel cells, because of higher efficiencies and lower fuel oxidation temperatures, emit less carbon dioxide (CO₂) and nitrogen oxides (NO_x) per kilowatt hour (kWh) of power generated than turbines or engines that use a combustion process. The overall air emissions are lower for fuel cells, but the difference is not significant for sulfur dioxide (SO₂) or particulates.  

If fuel reforming is done on site, heat produced from the fuel cell process powers the reformer. If the re-forming is done off site, the resultant pollutants would be produced off site, and there would be additional pollutants from transporting the hydrogen to the fuel cell site. Unlike gas fired combustion turbines and combined-cycle units, noise and vibrations associated with fuel cells are practically non-existent because the fuel cell itself has no moving parts. 

5 kW Fuel Cell System, Manufactured by PlugPower, Installed at a USDOD Facility.

Five 200 kW Phosphoric Acid Fuel Cells Installed at the Anchorage Mail Processing Center in Alaska.

Photovoltaics

Photovoltaics directly convert sunlight into electricity through the use of photovoltaic cells, which are grouped together to form a panel. Photovoltaic panels can be used in small groups on rooftops or as part of a substantial system for producing large amounts of electrical power. The amount of energy produced by a photovoltaic system depends upon the amount of sunlight available. The intensity of sunlight varies by season of the year, time of day, and the degree of cloudiness.

Currently, photovoltaic generated power is less expensive than conventional power where the load is small and the area is too difficult to serve by electric utilities. Recent breakthroughs may reduce the cost of producing electricity with photovoltaic systems to 10 to 12 cents per kWh or lower. This compares to 3 cents per kWh for fossil central station power generation.

While further advances in solar technology are likely, some technologies are available today. As a result of private and government research, photovoltaic systems are becoming more efficient and affordable. With continued improvements, it is likely that photovoltaic technologies will become increasingly cost competitive with conventional generation sources. Compared to traditional methods of electric generation, photovoltaic systems have few environmental concerns.

12 kW PV Installation at TDL Electronics near Racine, WI

Microturbines

Microturbines are small gas turbines, self-contained sources of electricity (generally less than 300 kW) and heat (typically hot water) that provide a controlled source of on-site power. They can operate in parallel with other units or operate alone. Efficiencies vary from 24 percent to 60 percent depending on initial cost and the utilization of waste heat. Natural gas is the primary fuel utilized although some renewable applications for biogas are being pursued.

Microturbines date back to the 1950-1970 time period, when the automotive market started looking at gas turbine products. Stationary market interest was spurred by the Public Utility Regulatory Policy Act (PURPA) in the mid-1980s and accelerated during the 1990s.

Microturbines are a developing technology that hold the promise of higher efficiencies and potentially lower operating cost. Items that will affect the market for microturbines attractive to widespread development include:

• The initial price as microturbines move down the price volume curve from their current initial cost
• Reliability and cost of fuel supply
• The degree of customer attraction to a “high tech” product
• The extent to which units demonstrate long-term operating reliability

The primary environmental concerns with microturbines are noise and air emissions. CO₂ and other emissions from natural gas (NG) are relatively low compared to other fossil fuels.

30 kW Capstone Microturbine with Cover Removed.

Multiple 30 kW Capstone Microturbines Installed at Sauk County Landfill.

Internal Combustion Engines

At present, the use of internal combustion (I/C) engines outnumber all other distributed generation technologies combined. Several factors make I/C engines attractive.

• Low initial cost
• Proven technology
• Readily available infrastructure for purchase/repair
• The small footprint required for a given amount of energy

I/C engines can be fueled by diesel, gasoline, methane, or natural gas. Historically, most I/C engines used for electric power generation, use diesel fuel. Gasoline engine generator sets are usually not selected for DG applications because of the reasons outlined in the efficiency section. They are available as emergency generators where limited run time of less than 100 hours per year is expected.

Natural gas engines have emerged as a competitive prime mover for the power generationmarket. Sales have grown from 4 percent to over 20 percent of all engine sales for generation since 1990. The use of natural gas in I/C engines is expected to increase at the expense of diesel and small turbines. This growth is due to improvements in cost, efficiency, reliability, and emissions. The additional direct cost for an engine equipped to burn natural gas or methane, rather than diesel fuel, is justified if the unit operates more than 2,000 hours/year.

Noise levels and air emissions from diesel engines are environmental concerns.

Waukesha Engine-Genset Installed at the Wastewater Treatment Plant in Madison, WI.

1.6 MW Caterpillar Engine-Genset Installed at the Rodefeld Lanfill in Dane County, WI.

Wind Turbines

Wind energy is converted to electricity when wind passes by blades designed like those of an airplane propeller mounted on a rotating shaft. As the wind moves the blades, the rotation of the shaft turns a generator that produces electricity.

Three factors affect wind machine power:

• the length and design of the blades,
• the density of the air, and
• wind velocity.

Blades are shaped and positioned to take advantage of different wind velocities so that, depending on design, one wind machine may produce power in a different range of wind velocities than another. Power output is directly proportional to the length of the blades. Cold air is denser; therefore, it has more force, or ability to turn the blades. A wind machine in Wisconsin’s cold, dense winter air can produce up to 20 percent more than the same machine exposed to the same wind speed but warmer air.

Since output is proportional to the cube of the wind velocity,1 the speed of the wind is critical for the cost-effective operation of wind machines. P=1/2DAV³ (P=power produced; D=air density; A=swept area of the turbine blades; and V=the velocity of the wind in miles per hour.) Generally, the higher a wind turbine is mounted, the more wind it will encounter.

Wisconsin electric utilities have established a comprehensive statewide wind resource assessment program (WRAP). This program was ordered by the Public Service Commission to encourage wind power development in those areas of the state with the best wind energy potential. For a three-year period, wind speed and direction will be recorded at 13 sites and at 10, 25, 40, and 60 meters above ground level. The information from WRAP is available to the public through the Focus on Energy webpage at www.focusonenergy.com.

Wind energy can have both positive and negative impacts on the environment. Like photovoltaics, wind power does not create air pollution. The primary concerns associated with wind power are potential effects on bird and bat mortality and aesthetics.

Vestas V27 225 kW Wind Turbine.

Combined Heat and Power

Providing both electric power and heat from a single source is called combined heat and power (CHP), also known as cogeneration. While separate heat and power systems are often only 33 percent efficient (67 percent of the fuel energy in wasted), CHP can be 60 to 80 percent efficient by capturing and making productive use of the waste heat on-site.

CHP can be used with fuel cells, micro turbines, small gas turbines, and internal combustion engines. Heat recovery lends itself particularly well to small-scale generation because small-scale generators can be located next to heat loads such as green houses, health clubs, food processors, and commercial laundries.

CHP can provide power and steam while burning one third less fossil fuel than would be the case without heat recovery. This means that CHP reduces the adverse effects of burning fossil fuels to power small generators. At the federal level, the DOE has initiated a program to double the use of CHP nationwide by the year 2010.

Heat Recovery from Coolant and Exhaust at an Engine-Genset Installation.