More Than Heat and Power: A Fresh Look At Cogeneration
Michael A. Devine
Caterpillar, Electric Power Division
Gas Product Marketing Manager
August 2013
INTRODUCTION
Cogeneration with gaseous-fueled engine-generators has delivered substantial benefits for many years. In Europe and North America, it provides extremely cost-effective electricity and heating in numerous commercial and industrial settings. In Asia and elsewhere in the developing world, it 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.
Today, the future of cogeneration looks brighter than ever. Shale gas development made possible by hydraulic fracturing (fracking) has driven North American natural gas prices down to levels not seen since the 1990s. Average wholesale prices fell 31 percent in 20121, and recent prices generally have ranged from $3 to $5 per MMBtu. This helps enable attractive payback on the front end of cogeneration projects. The longer-term outlook is favorable as well: Current forecasts call for natural gas prices to increase by just 2.1 percent per year through 20352.
Meanwhile, utility electricity prices continue to escalate, and advances in technology are pushing the efficiency boundaries of reciprocating engine-generators – adding to the appeal and financial return of engine systems. As a result, a widening variety of cogeneration applications have moved squarely into the mainstream. 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 way of absorption chillers
- Carbon dioxide from purified exhaust
In other words, a single engine-generator can produce two, three or four useful outputs at once. With today’s generating technologies, electrical efficiencies up to 45 percent and total resource efficiencies upwards of 90 percent are achievable. And 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.
HEAT SOURCES
Modern lean-burn gas-fueled reciprocating engines are rich sources of heat. A great deal of heat otherwise wasted can be extracted for productive uses, depending on the user’s heat requirements (Table 1).
Engine exhaust provides by far the highest temperatures and the greatest heat output. The typical exhaust temperature is about 460° C (860° F). Exhaust heat can generate intermediate-pressure steam for purposes like boiler feedwater heating, and lowpressure steam for processes like sterilization, pasteurization, space heating, tank heating, humidification, and others. In addition, supplemental firing with natural gas can increase exhaust temperatures and heat output to produce steam at greater volumes and pressures, creating even more possibilities.
Heat can also be extracted from the engine jacket water to produce warm or hot water at temperatures of up to 99° C (210° F) for space heating and a broad variety of industrial processes. Lower quality heat is available from the oil cooler (if it is not already included in the jacket water circuit) and the second stage of the aftercooler to meet additional low quality hot water applications.
DEPLOYING HEAT ENERGY
Hot water and steam are the classical engine outputs in cogeneration systems, but they are not the only ones: They can be converted to other forms to suit additional purposes:
Heated Air
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.
Chilled Water
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 93° C (200° F); COP typically ranges from 0.7 to 0.9. More complex doubleeffect units, activated at 175° C (347° F), deliver higher COP (1.05 to 1.4), although at greater up-front cost.
Heat-recovery systems can be configured to deploy some heat for water and steam production and the balance to absorption chillers – a concept called tri-generation. Alternatively, systems can produce space heat in winter and air conditioning in summer. For example, in São Paulo, Brazil, energy services company, Ecogen, operates an energy plant that serves the Rochavera commercial office complex that encompasses 8,000 m2 (86,111 ft3) and several office buildings. The energy plant uses a total of four Caterpillar G3520C gas generators sets for a total of 6.4 MWe with jacket water and exhaust gas heat exchangers that capture the engine’s thermal energy and transfer it to a common water circuit that is fed to four 540 ton (TR) rated hot water absorption chillers. The absorption chillers operate in parallel with two 340 ton electric chillers, three 450 ton electric chillers, and a 320 ton natural gas-fired chiller to maintain the facility’s cooling needs. All electrical power generated from the Cat generator sets is then fed through paralleling switchgear and distributed to the campus. Ecogen also employs two G3512B diesel generator sets, each standby rated at 1,500 ekW to provide emergency or peaking power to the facility.
More Electricity
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. Here 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 percent and improve electrical efficiency by 5 to 6 percentage points.
Shanxi Jincheng Anthracite Coal Mine Mining Company operates one set of facilities in Jincheng, Shanxi, China. The company collects coal gas from underground coal seams to power four separate power generation plants, each housing fifteen Cat G3520C high voltage generator sets that are integrated via Cat paralleling switchgear and controls. Each powerhouse is designed with a combined-cycle system that recovers waste exhaust heat to power a 3,000 kW steam turbine. The result of this heat recovery scheme is an additional 12 MW of electrical power to the local grid, which is equivalent to 10% of the power plants’ 120 MW total electrical output.
More Heat
Heat pump technology can extract useful heat from lower-quality heat sources: 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 percent (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 heat value and the high heat value of a fuel).
Using exhaust CO2
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 percent. Exhaust gas can also 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 quad-generation.
At the Eric van den Eynde Greenhouse in Kontich, Belgium, 95 percent of the electricity generated is sold to the local electric utility based on the premium paid for high-efficiency power. The jacket water, exhaust, first stage aftercooler, and oil cooler heat from two G3516A (1,070 ekW each) generator sets and one G3520E (2,070 ekW) gas generator set are captured and stored in the form of hot water. The hot water is used to stabilize the greenhouse temperatures. Water with a temperature of up to 95° C (203° F) is stored in a 1,200 cubic meter (1,569 cubic yard) tank which acts as a thermal battery. The tank provides hot water at 45° C (113° F) via metallic tubes in the greenhouse. The temperature is maintained between 19° and 21° C (66° to 69° F) throughout the year.
