By Carol Brzozowski
Cogeneration, or combined heat and power (CHP), uses a heat engine or power station to generate electricity and heat for onsite uses. Trigeneration, or combined cooling, heat, and power (CCHP), is the simultaneous generation of electricity and useful heating and cooling as a result of the combustion of fuel or the action of a solar heat collector.
According to Elite Energy Systems, CHP principles have been around since the late 1800s when Thomas Edison used them to power the world’s first commercial power plant. But as the need for electricity spread to rural parts of the country, government regulations were put in place to promote centralized power plants managed by regional utility companies.
Decentralized sources of energy such as CHP were discouraged, culminating in a law barring non-utilities from selling power. Thirty years ago, CHP once again became a favorable form of energy production after Congress acknowledged large power plants, though a convenient form of electricity production were inefficient in capturing the energy they produce and contributed to air pollution, according to Elite Energy Systems.
Advances in cogeneration and trigeneration technology are pushing the efficiency boundaries of reciprocating engine-generators, adding to the appeal and financial return of engine systems for private and public entities that are used to using large amounts of power for energy, cooling, or heating.
Michael Devine, the gas product marketing manager for Caterpillar’s Electric Power Division, points out in a white paper that cogeneration with gaseous-fueled engine-generators has delivered “substantial benefits for many years” in Europe and North America, where it provides cost-effective electricity and heating in numerous commercial and industrial settings.
In Asia and elsewhere in the developing world, cogeneration provides a steady source of electricity where utility power reliability and quality are inconsistent, while also delivering heat for process industries that help drive economic growth.
These days, cogeneration is receiving a boost through shale gas development, driving North American natural gas prices down to levels not seen since the 1990s. Average wholesale prices fell 31% in 2012, and recent prices have ranged from $3 to $5 per MMBTU (1 million BTUs). This helps enable attractive payback on the front end of cogeneration projects, Devine points out. The long-term outlook is just as favorable, with current predictions for natural gas prices to increase by just 2.1% per year through 2035 against the backdrop of escalating utility electricity prices.
“As a result, a widening variety of cogeneration applications have moved squarely into the mainstream,” says Devine. “Cogeneration today goes well beyond the classical picture of simultaneously generating electricity and hot water or steam.” Today’s usable engine outputs also can include heated air, chilled water produced by absorption chillers, and carbon dioxide (CO2) from purified exhaust.
|Photo: Energy Concepts
A 60-ton HeliChiller solar thermal refrigeration/ air-conditioning system
Varieties of Configurations in CHP, CCHP
“A single engine-generator can produce two, three, or four useful outputs at once,” says Devine. “With today’s generating technologies, electrical efficiencies up to 45% and total resource efficiencies upwards of 90% are achievable. Cogeneration systems do not necessarily need to operate full-time at full load to be cost-effective; low-cost and low-intensity configurations can bring attractive returns in many settings.”
Lean-burn gas-fueled reciprocating engines are rich sources of heat. Heat that is otherwise wasted can be extracted for productive uses. Engine exhaust provides the highest temperatures and the greatest heat output.
The typical exhaust temperature is about 860°F. Exhaust heat can generate intermediate-pressure steam for purposes such as boiler feed water heating, and low-pressure steam for processes such as sterilization, pasteurization, space heating, tank heating, humidification, and others.
More possibilities are created by supplemental firing with natural gas, which can increase exhaust temperatures and heat output to produce steam at greater volumes and pressures. Heat can also be extracted from the engine jacket water to produce warm or hot water at temperatures of up to 210°F for space heating and a variety of industrial processes. Lower-quality heat-if not included in the jacket water circuit-is available from the oil cooler and the second stage of the after-cooler to meet additional low-quality hot water applications.
Steam or hot water can be passed through heat exchangers to create hot air to feed equipment such as kilns and dryers. The heated air is mixed with fresh outside air to enlarge the volume and enable precise temperature control.
Steam, hot water, or exhaust can be passed through absorption chillers to produce cold water for space or process cooling. Absorption chillers use heat instead of electricity as the energy source. Their efficiency is measured by coefficient of performance (COP). Simple, relatively low-cost single-effect absorption chillers are activated at temperatures as low as 200°F; COP typically ranges from 0.7 to 0.9. More complex double effect units activated at 347°F deliver higher COP of 1.05 to 1.4, although at greater upfront cost.
Heat-recovery systems can be configured to deploy some heat for water and steam production and the balance to absorption chillers in a trigeneration setup. Alternatively, systems can produce space heat in winter and air conditioning in summer.
Where a site requires continuous prime power and has little or no heat load, engine exhaust heat can be used to boost electrical output through the organic Rankine cycle. The exhaust, typically from multiple engines, feeds a boiler that converts a working fluid to vapor, which in turn passes through a turbine. This configuration, similar to combined-cycle electric power plants, can boost generating capacity by roughly 10% and improve electrical efficiency by five to six percentage points, says Devine.
Heat pump technology can extract useful heat from lower-quality heat sources such as the engine aftercooler circuit, residual heat from the exhaust downstream from the exhaust heat recovery boiler, and even radiant heat from the engine block. This heat can be used to preheat the engine jacket water heat recovery circuit or for low-temperature space or process heating. Such heat pump installations actually can raise overall system thermal efficiency to slightly greater than 100%-based on the fuel low-heating value-with the recovery of heat lost due to the latent heat of vaporization in the combustion process, the difference between the low and high heat value of a fuel.
Beyond heat recovery, carbon dioxide in the exhaust gas is a usable byproduct of power generation. Engine exhaust rich in CO2 can be cleaned in a catalyst reactor, cooled and fed to a process. “In greenhouses, for example, CO2 fertilization helps crops grow faster, improving yields by 10 to 20%,” says Devine. “Exhaust gas also can provide a low-cost source of CO2 for industrial applications or even for carbonation in soft drink bottling. Taking efficiency to the ultimate level, a single generator set can deliver electricity, space or process heating, space or process cooling, and CO2- a concept known as quadgeneration.”
Heat Recovery From Many Processes
Cogeneration is not limited to highly engineered systems that maximize production of both electricity and heat, Devine points out. “Simple and well-conceived heat recovery can improve the economics of many electric power projects with only a modest additional investment,” he says. “Almost any application that entails roughly 1,000 or more annual operating hours offers potential for economical heat recovery. The only firm requirement is that the value of heat recovered outweighs the added cost of the heat-recovery and control mechanisms.”
Heat recovery from the engine cooling circuit is simple, Devine says.
“A shell-and-tube or plate-and-frame heat exchanger can produce water at 180 to 210 degrees Fahrenheit, depending on the engine jacket water temperature,” he says. “This water can serve purposes that include space or domestic water heating, light production process heating, and boiler condensate or make up water preheating, as well as air conditioning, process cooling, and desiccant dehumidification.” In each case, the recaptured heat displaces some costs for fuel or utility electricity, Devine says.
“To the extent that the recovered heat supports energy needs during times of peak electric load, total demand, and, thus, demand charges are also reduced,” he says.
Examples of low-intensity, limited-duty cogeneration include as follows.
- Commercial real estate: Office buildings can cost-effectively operate generator sets during business hours, avoiding utilities’ highest time-of-use rates. If heat recovery from a jacket-water heat exchanger can partially offset the cost of fuel for space heating, water heating, or dehumidification, then return on investment improves.
- Light industry: A small- or mid-sized manufacturer with an onsite generator set could install a heat exchanger in the engine cooling system loop, with a thermostatically controlled diverter valve to regulate the flow to the in-plant load, thus cost-effectively satisfying a variable hot water requirement.
- Hospitality: Hotels can readily use heat recovery for domestic hot water, laundries, kitchens, or swimming pool heaters. In summer, the recovered heat can power absorption chillers or desiccant dehumidifiers for space conditioning.
- Food processing: Food producers can recover exhaust and jacket water heat to create low-pressure steam for light process loads such as cooking or raising dough, or to produce hot water for cleaning and sanitizing. Depending on the size and character of the heat load, such systems can be cost-effective in single- or multiple-shift service, even if heat demand is cyclical or seasonal.
|Photo: Energy Concepts
ThermoCharger turbine inlet air chiller also known as ARCTIC
Cogeneration With Gas
Advances in gas engine technology have been essential to progress in cogeneration, says Devine. “By nature, gas engines serve well as isolated or grid-connected onsite power sources,” he adds. “They offer high power density, low first cost per kilowatt, and quick, simple installation. Fuel and operating costs are competitive, emissions are clean, and service is readily available with an abundance of trained and qualified technicians worldwide.”
The basic engine technology is highly reliable, Devine points out.
“Uptime can approach 98%, including all maintenance and repairs,” he says. “Engines readily handle full or part loads, tolerate varied altitudes and ambient conditions, and can be brought online quickly. The latest engines develop high power output in footprints up to 50% smaller than traditional units, fitting easily in small engine rooms or containerized power plants.”
Gas engines are well suited to operate in cogeneration service on refined natural gas and propane, as well as on fuels of variable heating value and purity, Devine says.
Landfill gas, agricultural biogas, and wastewater treatment plant digester gas may produce exhaust containing corrosive compounds, requiring stainless steel surfaces in exhaust gas heat exchangers, but no special modifications are required for heat recovery from liquid cooling circuits. Other specialty gases like coke gas and coalmine methane are also viable cogeneration fuels.
Advances in Engine Technology
The latest engines, proven in multiple applications all over the globe, use a variety of digital microprocessor-based monitoring and control technologies that include the following.
- Air/fuel ratio control based on charge air density, maintaining NOx (nitrogen oxide) emissions within the tightest available tolerance under all ambient and load conditions irrespective of changes in air temperature and humidity
- Air/fuel ratio control based upon Total Electronic Management systems that optimize generator set performance, measuring each cylinder’s temperature and adjusting combustion to minimize fuel consumption and engine emissions, and prevent detonation
- Control system strategies that can be configured to control functions beyond the engine itself, such as radiator motors, electrical breakers and systems, and even a complete cogeneration plant
- Detonation sensing by individual cylinder with automated control to retard timing if detonation occurs. These engines are designed with intake systems that enable efficient air flow and minimize heating of charge air, increasing the air/fuel charge to the cylinders for optimum performance. An open-chamber cylinder design and low-pressure fuel system [0.3535 kPa (0.5 to 5 psi)] eliminate the need for a fuel compression skid, and the skid’s subsequent cost and ongoing operational expense.
Other advances in engine technology include as follows.
- Optimized air and exhaust flow: Both the inlet and the exhaust system are tuned to enable highly efficient, laminar flow. A technology called Pulsed Energy Advanced Recovery Line (PEARL) uses flow-optimized exhaust pipes that convey a constant exhaust mass flow to the turbochargers. Each PEARL module evacuates the exhaust of two cylinders. Exhaust flow is timed so as to keep the turbocharger spinning at the optimum speed over the engine’s entire operating load range. Precise ignition timing-automatically adjusted by cylinder for fuel quality changes-augments the entire process.
- Miller cycle: This adjustment in the combustion cycle by itself increases fuel efficiency by about one percentage point. The Miller cycle differs from the more traditional Otto cycle in that the intake valves close not when the piston reaches bottom dead center, but at typically 10 to 15 degrees before bottom dead center in the early inlet close version. As the piston continues down with the intake valves closed, the air/fuel mixture expands and cools, increasing the detonation margin. This enables a higher compression ratio of 14:1 or 15:1, versus 11:1 or 12:1 for the Otto cycle, and is designed for higher fuel efficiency.
- High-energy ignition: Pre-chamber spark plug technology, improved with optimized plug geometry. The spark plugs admit air and fuel through small orifices and upon ignition eject flame through the same orifices. High-energy ignition coupled with pre-chamber plug technology enables the engine to operate on a significantly lean fuel mixture without risking lean misfire, thus sustaining high power output and low emissions over extremely long life spans.
In recent years, an alternative approach to off-the-shelf engines has emerged in the form of individual generator sets customized to fit the application, Devine says. “Rather than purchase an engine and accessory package based on published ratings and a price list, users provide a sample of the fuel expected to be used, describe the ambient conditions and altitude, and specify the application and key operating objectives such as top fuel economy, lowest emissions, block loading capability,” he says, adding that the manufacturer then custom-designs a gas engine-generator system to fit those criteria.
In the range of customization options available, application engineers can select from a variety of compression ratios, pistons designed for specific fuel types, different turbochargers and nozzle ring configurations, and site-specific air system operating and engine timing maps.
“The customized units come with further advances in engine technology that expand the limits of engine control and reach new heights of efficiency-up to 44% electrical efficiency in the generator sets alone and up to 90% total plant efficiency in cogeneration or trigeneration service,” says Devine. The technology advances contribute to a new paradigm in maintenance intervals, he adds. “The intervals for spark plug replacement and oil changes are designed to be the same at 4,000 hours-some six months of continuous operation, or more than double the interval expected with traditional technology,” says Devine.
After about eight hours of planned downtime for service, the engine is ready to run for another six months, he adds. Cylinder head overhaul intervals are at approximately 32,000 hours and major overhaul intervals are approximately 64,000 hours with fuels containing contaminants or impurities, dictating shorter intervals. Operating costs are further reduced by lowering oil consumption: engine technology innovations have cut oil consumption by as much as half, a savings of 500 gallons and $10,000 per year on a 20-cylinder engine-generator rated at 2 MW, says Devine.
The Economic Question
Every cogeneration project comes down to a question of economics, Devine points out: “Will savings on energy costs and revenue from electricity generated provide adequate return on the investment in equipment?” In general, he says, the outlook is most favorable where:
- The utility electricity cost is relatively high.
- The fuel price is relatively low.
- The system will operate with a high electrical and heat load factor.
- Electric and thermal loads coincide during a typical day.
- The site requires high reliability and power quality.
- The cogeneration system can double as a standby power source.
The availability of a low-cost “opportunity fuel” such as anaerobic digester or landfill gas generally improves the economics, Devine says. In particular, digester gas-fueled cogeneration is a key contributor in a growing quest for energy self-sufficient wastewater treatment plants.
In exploring project design alternatives, engine fuel efficiency is one of many considerations. “For example, capacity factor-the percent of total theoretical output the generators actually achieve-may far outweigh fuel savings,” he says. “Furthermore, if high efficiency comes at the cost of increased downtime from more frequent maintenance or engine sensitivity to fuel variability or quality, or if performance is degraded at higher ambient temperatures, then project economics are compromised.”
Other engine capabilities such as low emissions or fast response to block loads may also be more important than fuel economy in some settings, he adds.
For the development of most CHP projects, it takes specialized expertise to determine how best to deploy the equipment on a site and to evaluate the project economics. Devine advises that it’s best to check for fatal flaws-such as physical or cost barriers that would make the project impractical-before investing in a full feasibility study. For example:
- Will it be impossible or extremely expensive to acquire an air-quality permit?
- Will it be difficult to secure a wastewater permit to discharge cooling water or exhaust condensate?
- Is the available space on the site too small to accommodate the engine and heat recovery equipment?
- Will the natural gas service need a costly upgrade to deliver the necessary fuel?
- Is the facility’s electric power infrastructure inadequate to distribute the generated power?
- If the project success calls for excess electric energy generated to be exported to the grid, do local utility policies preclude such sales?
- If exporting power to the local utility is possible, is the power purchase price too low to allow for profitable operation?
“If the answer to one or more of these questions is “˜yes’, then you will want to consider the impact of resolving the issue on the project economics and the impact that the resolution will have on project feasibility,” says Devine.
If nothing stands in the way at that point, the next step is a high-level look at project economics, Devine says. “This involves estimating and stacking the component costs and savings-or revenues-per kilowatt-hour to arrive at the net benefit,” he says, adding that a local power-generating equipment dealer can be a good source of reasonable, experience-based estimates. The major cost components are:
- fuel, typically the largest item at 60 to 80% of project operating cost, unless an opportunity fuel is available
- capital recovery: principal and interest on the equipment investment
- operations and maintenance-staffing, components and supplies for daily operations, periodic service and repairs
From the total of these costs, the thermal credit-the economic value of the recoverable heat-is deducted. “This is typically done by looking at the cost per kilowatt-hour of replacing the existing thermal system with a higher-efficiency system,” he says. “If the resulting total net cost per kilowatt-hour is significantly lower than the retail price of electricity, including demand charges, then the project probably merits further investigation through an engineering study.”
In the analysis, it is essential to understand utility electricity costs in detail, including on- and off-peak energy prices, on- and off-peak demand charges, standby charges, and any non-availability penalties and existing or pending cogeneration incentives from the local utility or government entities, Devine points out. Ideally, a project should be developed in a cooperative relationship with the local utility, he says.
“For example, a project that reduces the user’s overall energy costs while helping the utility limit peak demands on its grid provides a win-win and may even benefit from utility incentives that enhance economic payback,” says Devine.
Another benefit of a cogeneration system is that it can function as a standby power source, although not for life-safety electric loads such as hospital operating suites where diesel standby with onsite fuel storage is required, Devine points out. “In today’s emission-conscious environment, demand is growing rapidly for gas-fueled standby generators,” he says. “Such systems, installed for utility-parallel cogeneration and operated for extended hours continuously provide electric power security while also generating revenue.”
Devine says few organizations have the in-house expertise to plan and implement cogeneration projects. Equipment suppliers and consultants can provide vital support from project planning through design, financing, construction, operation, and maintenance. “An appropriate project partner should have a deep understanding of cogeneration and a track record for completing profitable projects,” he points out.
CHP Modules-Standard and Custom Built
Elite Energy Systems provides pre-engineered “standard” CHP modules, as well as custom-built solutions for commercial and industrial energy users; these are powered by Caterpillar engines with multiple fuel options including natural gas, biogas and LPG (liquefied petroleum gas). The most common project sizes are in the 150-kWe to 2,000-kWe size range, with multiple units extending beyond that range.
Elite’s systems incorporate synchronous or inductive generators, seismic isolation, sound attenuation, ultra-low emissions, sound-eliminating air ducts, integrated design, stainless exhaust, rain-tight construction, removable door and roof sections for maximum serviceability, on-board primary containment, separated controls cabinet, battery charger, stored energy breaker, and Internet connection for remote system monitoring.
Elite designs CHP systems that focus on the optimum use of the recovered waste heat created while generating electricity. The recovered heat comes from the engine coolant circuit and exhaust gases, and is effectively “free energy.”
The most common applications focus on providing heat for process heating such as domestic hot water and swimming pools. The use of the recovered waste heat is an alternative to burning fuel in a boiler or using electricity for heating. CHP systems also can be designed to generate chilled water using an absorption chiller that uses the recovered waste heat as its energy source. In addition, CHP systems can be designed to generate low-pressure steam.
For agriculture, Elite also designs, manufactures, and owns anaerobic digester systems that operate on this formula: six digesters plus two CHP modules which produce 300 kW of electricity as well as organic soil amendment and compost tea. These modular systems are sized for about 1,500 lactating cows. Elite operates its own manufacturing and testing facilities to ensure its systems meet or exceed the demands of the customer site requirements while meeting stringent air emissions regulations.
The company has designed systems to operate in both parallel and island modes and maintains interconnection agreements with many publicly held and private utilities, including net metering installations for power sell back to the grid. The operation of Elite’s CHP systems are remotely monitored, interrogated, and controlled using control system technology designed with an interactive interface to enable the user to communicate with the CHP system 24/7.
One project in which Elite has been involved is the John Muir Medical Center, a 259-bed acute care facility in Concord, CA. In summer 2010, the company was commissioned to design a system to help the medical facility incorporate green building technologies that would include energy efficiencies in its expansion project.
The system consists of three 250-kWe CHP modules and one 150-kWe CHP module for a total of 900 kWe powered by three Caterpillar 3412 engines and one Caterpillar 3406 engines, all running on natural gas. Heat from the engine exhaust is captured with jacket water to provide hot water to the hospital, and an absorption chiller also provides seasonal cooling to the facility.
The system was created to provide a number of benefits to the medical center including more than 86% total fuel efficiency in contrast to 27% for the utility, displacement of electricity from the utility for similar costs with distributed generation, the creation of additional thermal energy at no cost, the reduction of power demand charges from the utility, the enhancement of power quality and reliability, an eligibility for energy efficiency grant programs, redundancy for electrical and thermal energy, and lowering greenhouse gas emissions and carbon footprint.
Don Erickson, president of Energy Concepts Company, points out that much of the global energy demand is thermal-either for heating or cooling. “Too often, those thermal demands are met wastefully,” he says. “For example, by using prime fuel to produce low grade heat, by rejecting waste heat, or by supplying thermal needs from electricity.”
Energy Concepts is commercializing heat-activated cycles powered by otherwise wasted availability instead of prime fuel. Applications range from hot water heat pumping, chilling, industrial refrigeration, LNG (liquefied natural gas) production, light ends recovery (flare gas elimination), ice production, desalination, and others, including power production from waste heat.
“Waste heat has been termed the world’s greatest under-exploited energy resource, and Energy Concepts technology derives greater value from waste heat,” points out Erickson. “Our technology is a key enabler for more economic CHP systems. We offer large-scale savings at two- to three-year paybacks.”
Energy Concepts designs, develops, and manufactures energy-efficient, heat-activated absorption systems and fluid contact equipment. The ammonia absorption systems produce refrigeration, heat pumping, space conditioning, and/or power. There is a wide range in system capacity, from small residential to large industrial equipment powered by waste heat, solar heat, or prime heat.
Those systems that upgrade waste heat to achieve greater value than simple heat recovery include the ThermoSorber thermal heat pump, ThermoCharger turbine inlet air chiller (also known as ARCTIC), engine interchiller, ThermoChiller waste heat powered refrigeration/air-conditioning system, and HeliChiller solar thermal refrigeration/air-conditioning system. A chilling economizer for steam boilers, heat recovery steam trap, condensate tank heat recovery, steam heat pump, ISAAC solar icemaker, plus new heat exchanger geometries and new distillation and desorption column configurations also advance the potential of waste heat for numerous processes. Dual function absorption cycle power and refrigeration from waste heat, absorption power cycle, steam-ammonia power cycle, and liquid desiccant chiller are all being developed by Energy Concepts.
In April 2011, Energy Concepts Company rolled out its new “standard design” Thermal Heat Pump (THP). The initial production model was designed with a heating capacity of 175-kW (600,000 BTU/hour) and 70-kW chilling capacity (20 tons) to be powered by 105 kW (360,000 BTU/hour) input heat and 2 kW electricity.
Thermal Heat Pumps are powered almost entirely by heat rather than by electricity, a technology that conserves gas and electricity. It does not exacerbate peak electric demand in summer and winter and is designed to reduce demand peaks. Waste heat, exhaust, or solar thermal heat can power the THP, which offers a return on investment (ROI) averaging two to three years.
The Helisorber is a recent addition to the company’s line-up and serves as a solar chiller/thermal heat pump. It is designed to double the useful output from a solar thermal collector. For example, supplying 100 kW of thermal heat from a collector to the Helisorber will yield 75 kW of 44.6°F chilling, plus 175 kW of hot water heating to 140°F.
The California Energy Innovation Small Grant Program funded development of its prototype. That unit produces 87 kW (25 tons) of chilling and 215 kW of water heating from 130 kW thermal fluid heat input. The design has been optimized and scaled to refrigeration capacities from 10 to 200 tons (35 to 700 kW). In order to operate at this level, the chilling and heat pumping are produced simultaneously, so both must be used, stored, or lost. Additionally, the driving temperature must be 257°F or higher.
The technologies are used in several sections of a plant or mechanical system, including steam generation, steam distribution, refrigeration system (generation and use), hot water heating system (generation and use), distillation, and desorpti