Cogeneration as a Universal Power Solution

Cogeneration technologies are poised to play an increasingly important role in the energy mix of the future.
By Christian Mueller | June 29, 2017

Cogeneration technology, including combined-heat-and-power (CHP) systems and district heating-and-cooling (DHC) systems, offers many economic and environmental benefits compared to conventional methods of energy production. By simultaneously producing thermal and electric energy from a single fuel source, such as natural gas or biogas, the systems require less total fuel to produce the same amount of energy—and generate enormous cost-savings potential.

Because less total fuel is consumed, greenhouse gas emissions and other harmful air pollutants are also reduced. In fact, CHP technologies are estimated to reduce carbon dioxide emissions from new power generation by more than 10 percent by the year 2030.

Cogeneration of energy on-site can also support corporate environmental goals for sustainability and use of renewable resources, while simultaneously reducing the dependence on other regions or countries for imported energy. By increasing energy efficiency and helping to offset costs, cogeneration can give businesses a competitive edge.

Heat Sources Provide Opportunities
Cogeneration systems achieve more than 90 percent energy efficiency by extracting and using thermal energy produced during the generation of electricity, heat that would otherwise be wasted. Systems based on gas-fueled reciprocating engines include several potential heat sources, and the ideal use depends on aligning the heat requirements of the facility with the available heat sources on the system.

Exhaust gas, for example, reaches temperatures exceeding 450 degrees Celsius, hot enough to support an absorption chiller, which, in turn, creates cooling energy. Sources such as the lube oil, jacket water or high temperature air/fuel mixture, on the other hand, reach temperatures closer to 90 to 200 degrees C, which makes them ideal for industrial processes, drying processes, building heat and steam production. Other heat sources below 90 degrees C include the low-temperature air/fuel mixture, calorific value boiler and radiator, which are ideal for drying processes, underfloor heating and return temperature heat.

When using all the available heat sources, a cogeneration system from MTU Onsite Energy can achieve overall efficiencies of up to 96 percent—a best-in-class rating and a major improvement compared to conventional methods of energy production.

Site-Specific Factors
To ensure optimal performance and efficiency, several factors must be considered before installing a cogeneration system.

Methane Number
Most gasses are a mix of methane, hydrogen and other gas constituents. The methane number (MN) provides an indication of the gasses, tendency to knock—or combust prematurely—which can damage the engine. For example, pure hydrogen would have an MN of 0. A low MN signifies an extremely explosive gas with the potential to ignite before the spark plug fires, resulting in uncontrolled combustion. Pure methane (CH4), on the other hand, would have an MN of 100. Gasses with a high MN are less explosive, and therefore less likely to ignite before the spark plug fires, resulting in a more controlled combustion.

Natural gas has an MN of 80 to 90, making it ideal for controlled combustion. Gas composites (such as biogas) have an MN between 120 and 130. Understanding the knock resistance is important when specifying an engine for a gas-powered cogeneration plant.

Smart marathon runners train for the race conditions they’re going to run—especially if higher elevations are involved—otherwise, they’ll never be able to catch their breath. Similarly, the altitude of an installation site can significantly influence the power output of a cogeneration system. As elevation increases, air density decreases, and engines need air to breathe.

When specifying a gas-powered cogeneration system for a high-altitude installation, proper preparation and planning are essential to avoid operating below the engine’s maximum power rating. For instance, a gas-powered MTU Onsite Energy Series 4000 cogeneration system can operate at full load in altitudes up to 6,700 feet without any derating simply by adjusting its turbocharger nozzle ring, which essentially enables the engine to take deeper breaths.

Like elevation, the ambient temperature of an installation site can significantly impact power output. This is because air volume increases as temperature rises. In warmer climates, if the ambient temperatures exceed a certain point, it can become difficult to provide the necessary volume of intake air for the engine to perform optimally, resulting in lower power output.

Cogeneration system manufacturers frequently offer different equipment models to account for these variations. And in some cases, the equipment will be installed in a temperature controlled (air conditioned) room to help offset the impact of excessive ambient temperatures.

Air humidity and dew point must also be carefully considered when specifying a cogeneration system. Dew point is the saturation temperature for water and air, the point at which water droplets begin to condense and form. This measure of moisture varies according to atmospheric pressure and humidity. To ensure maximum power output, specifications must include an analysis of humidity and its related dew point temperature based on the climate of the installation site, along with other factors such as methane number and type of gas. Similar to temperature, cogeneration system manufacturers frequently offer different equipment models to account for these variations.

The need for highly efficient electricity, heating and cooling is universal. Cogeneration is a powerful solution that can generate cost savings and environmental benefits just about anywhere in the world. A wide number of applications utilize cogeneration systems, including office buildings, condos, shopping centers, schools, community pools and dairy farms. The key to a successful cogeneration project is proper specification and planning, which should take into account climate- and site-specific factors such as methane number, elevation, temperature and humidity, in addition to thermal and electric requirements. While determining if a facility is an ideal candidate for CHP is an extensive process, exploring the option is a smart move for any facility with simultaneous needs for heat, cooling and electricity. The potential gains of CHP are too great to be ignored.

Author: Christian Mueller
Gas Power Systems Sales Engineer,
MTU Onsite Energy