Are large generators more efficient?

Engine-generator sets come in all sizes, from the tiny boxed unit you take on camping trips to the enormous units that move container ships or supply power to utility grids. It&#;s a technology that has been tested by time and is well understood. Yet the reciprocating engine continues to improve in reliability, efficiency and control. Large stationary units are especially attractive for many electric power applications.

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Multiple engine types

Reciprocating engines operate on various types of cycles. The most common are the Diesel cycle, the Otto cycle and the Miller cycle. The Diesel and Otto cycles date to the 19th century, but have somewhat different operating characteristics. The Miller cycle is an enhanced development from the Otto cycle.

Diesel cycle engines use compression ignition with a liquid fuel, typically light oil, that is injected into the cylinder and explodes with power as the piston compresses the fuel-air mixture. The Otto and Miller cycles also uses a compressed mixture of fuel and air, but the mixture is ignited by a spark source. Otto cycle engines have historically been fueled with gasoline but are increasingly also using gaseous fuels. Miller cycle engines can also use a wider variety of fuels.

Growing interest in natural gas

With today&#;s growing abundance and availability of natural gas, and the long-term outlook for attractive pricing for this as fuel, we have seen a growing interest in engines burning this economically attractive and very clean fuel. Whether for powering automobiles or generators, natural gas engines are growing in popularity.

Natural gas can be used in both diesel and spark-ignition engines. When used in the diesel cycle, it is normally mixed with a &#;pilot&#; amount of diesel fuel to allow compression ignition. This pilot fuel might represent as little as 5% of the total fuel.

With spark ignition engines, natural gas can be used as the only fuel.

Choosing the largest practical size

As the size of a stationary generator increases, the engine efficiency usually increases as well. For this reason, industrial and institutional energy users benefit by choosing the largest-size engines suitable for their needs. Of course, this efficiency goal must be tempered with a need for reliability with redundant engines. Modern reciprocating engines are very reliable. Most owners are better off with two large engines rather than six smaller units.

Jamie Fox from Caterpillar notes, &#;It is important that the customer thoroughly compares apples to apples, as high efficiency comes at the price of higher capital cost and less operational flexibility. A good engine dealer should provide the customer with a manufacturer&#;s data sheet calculated at the exact site altitude conditions, with a graph or data to describe the power and efficiency derates of the engine at 77°F all the way up to the maximum temperature the engine can handle at a stated altitude.&#;

Fox also points out that one reason that larger engines are more efficient is they generally use the Miller combustion cycle rather that the traditional Otto cycle. &#;This is due to economic and standby response capabilities. The Otto cycle is inherently less efficient than the Miller cycle.&#; However, she adds that Otto cycle engines are unmatched in terms of quicker response to sudden load changes, tolerance to fuel contamination, and ability to operate independently of the utility.

Best results with a matched system

Fox indicates that a successful CHP project needs to be specified as a system and not as a collection of independent hardware. This means, she says, &#;Probably the best option for the end-customer is to purchase a turnkey CHP project from a power systems dealer or a very capable CHP packager. There are superb companies out there doing engineering, sourcing and integration of CHP projects. Good definition of scope and responsibilities is a must.&#;

Wartsila is another world leader in very large reciprocating engine-generator sets. The Company is headquartered in Finland and is a respected manufacturer of both marine reciprocating engines and large stationary engine-generator sets. Joseph Ferrari is the Business Development Manager for Wartsila North America, headquartered in Annapolis, Maryland.

Slower engine speeds

Ferrari notes that there are good reasons why larger engines are generally more efficient. &#;Larger engines have less combustion surface area relative to volume, leading to lower thermal losses (heat transfer to the cooling system) which in turn means more energy is available to push down the piston. So, the larger the cylinder, the more efficient the combustion process.&#; In addition, he says, &#;Theoretical engine efficiency is related to combustion length (in time) relative to degrees of crank shaft rotation. Therefore, slower speed engines tend to have higher efficiency.&#;

Wartsila medium speed (514-720 rpm) engines use very lean air-fuel mixtures and fast combustion as the key elements for lower emissions and high output per cylinder, therefore clean, high-efficiency combustion. Ferrari says this leads to Wartsila engine efficiencies being greater than any other simple-cycle gas-fired technology in the market. In some cases, engine efficiencies greater than 50% are achieved.

Total efficiency can exceed 90%

Ferrari adds that in situations where the heat from engine jacket water and exhaust are collected using heat exchangers, total plant efficiency of more than 90% can be achieved. The Wartsila SG engine-generator sets that operate on gaseous fuel only are available in sizes from 5.3MWe to 19 MWe (60 Hz). In addition to these gas-only sets, Wartsila offers dual fuel diesel generators that use as little as 5% of their input as light fuel oil.

One example of the growing attractiveness of large generator sets is the installation of multiple Caterpillar units in Markham, Ontario. Markham is a satellite community near Toronto that has seen significant growth in the past two decades. Community planners place emphasis on planned community density rather than urban sprawl. This makes Markham an ideal candidate for district energy, where local generation plants can also provide thermal energy for heating or other purposes. Markham District Energy (MDE) is the organization established to plan, maintain and operate the energy facilities.

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Ice storm experience influences selection

The decision by Markham planners to localize electric generation was in part influenced by a major ice storm that struck Eastern Canada in , causing large and long-term electric outages. One benefit of district energy is that the lights would stay on, even if there was a regional grid failure.

MDE selected natural gas-fired reciprocating engine generators as the method of choice. In , they designated Toromont Power Systems, the local Caterpillar dealer, to design, deliver and commission a 5.0 MWe plant at Warden Energy Centre, which went into operation in . Gas-fired boilers can supplement the hot water supplied by the engines as a thermal byproduct. Hot water is used for space heating and other applications throughout the district.

Continue choosing large engines

As MDE continued to expand its service area, in it added a 3.0 MWe Caterpillar engine-generator at its Birchmount Energy Centre and a 4.0 MWe Caterpillar unit at its new Burr Oak Energy Centre. MDE also operates a diesel standby energy plant at Markham Stouffville Hospitals for this anchor power and thermal energy customer.

Bruce Ander is the president and CEO of Markham District Energy. Regarding the importance of reliability, he states, &#;First and foremost, our core business is to create thermal energy, and reliability is our first priority. Whether the Cat® fleet is providing emergency power to our customers, or grid power, or thermal energy for our heating system, high reliability is the goal for MDE and our suppliers.&#;

High reliability

Reliability comes from several sources. Modern reciprocating engines take advantage of metallurgical advances and improved designs that minimize unexpected failures and extend times between necessary engine services. Because natural gas is a very clean fuel, this service interval time is further extended. Planned maintenance can be done at times of year when electric and thermal demand is lowest. And many of the larger engine sets have on-board diagnostic tools that alert operators of changing engine conditions so that maintenance can be scheduled rather than on an emergency basis.

Engines can be the best choice

Whether your operation is a school or university, an industrial campus, a healthcare facility, or something else, on-site electric generation with natural gas allows you to get more out of your energy dollar. Today&#;s large reciprocating engine technology is reliable and surprisingly energy-efficient. To get started, ask your natural gas supplier and your consulting engineer to help you consider this important option.

More info

Caterpillar Engine Generators

GE Jenbacher and Waukesha Engine Generators

Siemens Dresser Rand Engine Generators

Wartsila Stationary Power Plants

This article originally appeared in the Gas Technology Summer  issue.

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And, given current data, the need for generators is likely to grow. In the years between and , the average power outage in the United States increased in duration from just over 3 hours to 5½ hours, according to the U.S. Energy Information Administration.

Choosing a generator takes some thought, though, since they vary in type, price, and size.

Portable generators run the gamut from small recreational units that start at about $450 and can power a single appliance to large portable models that can power most of a home. And within those size categories, you have conventional generators and inverters that are quieter and more efficient, but also more expensive. (Take a deeper dive into the differences between inverter and conventional portable generators.)

The next option is a standby model&#;sometimes called a whole-house or home standby generator&#;that powers an entire house seamlessly and without frequent refueling. These models cost as much as $7,000 and require permanent installation.

How do you figure out which generator is right for your needs? Keep reading.

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