What are the different types of cogeneration prime movers?

With the increased interest in renewable energy sources over recent times, there has been a natural shift in the way in which people approach both energy production and usage. From solar to gas to wind and everything else available, the choices are becoming quite large and diverse.

Cogeneration

The same can be said for the ways in which energy is produced, with cogeneration being a front runner in the generation of electricity at higher efficiency rates. Cogeneration systems capture heat whilst producing electricity, with around 92% of energy put into a cogeneration plant being turned into electricity (instead of around 40% for traditional power plants). Prime movers are one of the major components when it comes to cogeneration, so we thought we'd give you a little information as to the best options around. If you have any questions, give Inoplex a call on 0448 307 282 and we would be happy to discuss any queries you may have regarding cogeneration prime movers.

Cogeneration systems

To begin, let's take a look at cogeneration systems. There are generally 5 basic components to a cogeneration system, including:

  • Prime Mover: examples include turbines (micro, steam and combustion), fuel cells and reciprocating engines.
  • Electric Generator: examples include inverters, induction and synchronous.
  • Heat Exchanger, otherwise known as a heat recovery system.
  • Absorption Cooling Unit (if trigeneration).
  • Control System, otherwise known as the control and operating strategy.

We are going to focus on the prime mover on this page, which constitutes the first part of a cogeneration plant, driving the electrical generator and producing heat as a by-product.

Cogeneration prime movers

Ensuring you have the correct prime mover at the correct size is a crucial component to any cogeneration arrangement, as it is the major part that runs the entire system. The major aspects you should consider when researching prime movers for your cogeneration plant include:

  • The levels of electrical output you require.
  • The temperature of the heat you require, sometimes referred to as the 'grade' of heat.
  • The heat to power ratio you need, otherwise referred to as the ratio of recoverable heat to electrical output.
  • The fuel/s that are available to you.
  • When referring to a waste heat recovery system, the required waste heat temperature available.
  • The form of thermal load, such as steam, chilled water or hot water, and the associated conditions (namely pressure and temperature).

Once you have taken a look at the above, some other points to consider include:

  • What noise levels are suitable or accepted in the area.
  • The space available for the prime mover.
  • Emission levels.
  • Maintenance requirements and what you are expected to upkeep.
  • Reliability of the system and of your maintenance schedules.
  • Engine turn down, or the ability to vary the levels of electrical and thermal output.

There are three common categories of prime movers in which to allpy these factors to:

  • Steam turbines.
  • Gas turbines.
  • Reciprocating engines.

There are also fuel cells and organic Rankine cycles, all of which have been looked at in detail below. All information courtesy of the NSW Government.

Cogeneration

Spark Ignition Reciprocating Engine (1500 rpm)

Spark ignition reciprocating engines come in sizes ranging from 50 to 4,000kW and are a quite common prime mover choice. Inoplex use the Ford Barra FG straight 6 engine for their cogeneration plants, which is a form of reciprocating engine.

Advantages

  • Start-up process is quick.
  • Low pressure gas fuel.
  • Electricity efficiency levels are high.
  • Can work on partial loads with high efficiency levels.
  • High grade heat (exhaust).

Disadvantages

  • Emissions levels are quite high.
  • If the heat produced is not used, the engine must be cooled.
  • Engine cooling produces low grade heat.
  • Relative maintenance costs are high.

Gas Turbine

Gas turbines are another prime mover choice, coming in sizes from 1,000 to 30,000kW.

Advantages

  • Very reliable.
  • Low emissions levels.
  • Low amounts of cooling needed.
  • High grade heat (exhaust).

Disadvantages

  • Performance declines over time.
  • Medium pressure gas fuel.
  • Efficiency levels for part loads is low.
  • Output lowers as ambient temperature rises

Fuel Cell

A fuel cell is a smaller prime mover, ranging in size from 5 to 1,400kW.

Advantages

  • Modular design.
  • No NOx emissions.
  • Low pressure gas fuel.
  • Noise levels are low.
  • Electrical efficiency is high over load range.
  • No direct air emissions.

Disadvantages

  • Durability is low.
  • Very high relative capital cost in relation to $/kW.
  • Performance declines over time.
  • If the heat produced is not used, the fuel cell must be cooled.
  • Low grade heat.
  • Gas fuel requires processing.
  • Power density levels are very low.

Steam Turbine

Steam turbines come in a huge range of sizes, with the smallest being 80kW and the largest being 500,000kW.

Advantages

  • Compact in size.
  • High levels of thermal efficiency possible.
  • Very reliable.
  • Runs on heat instead of fuel.
  • Configuration is quite versatile.

Disadvantages

  • Units of a smaller size are quite high cost.
  • Electrical conversion levels are low.
  • High grade heat is required.

Microturbine

Microturbines, as their name suggest, come in smaller sizes from 30 to 250kW.

Advantages

  • Start-up process is quick.
  • Very reliable.
  • Low number of moving parts.
  • Small in stature and weight.
  • Emissions levels are low.
  • Cooling is not required.

Disadvantages

  • Gas fuel is high pressure.
  • Performance declines over time.
  • Electrical efficiency is low.
  • The relative capital costs related to this prime mover are really quite high.
  • Low grade heat (exhaust).

Organic Rankine Cycle

Finally, the organic Rankine cycle comes in sizes from 30 to 10,000kW.

Advantages

  • Very reliable.
  • Low grade heat acceptable.
  • Runs on heat instead of fuel.

Disadvantages

  • The relative capital costs related to this prime mover are really quite high.
  • Low electrical conversion rates

Choosing a cogeneration prime mover

To make things a little easier when choosing a cogeneration prime mover, see the below 'tree' created by the NSW Government:

Your knowledgeable cogeneration specialists

If you have any questions regarding cogeneration prime movers, or would like to speak to one of our Inoplex team members regarding a cogeneration system for your business, give us a call on 0448 307 282 today.

Resources:

NSW Government (Cogeneration Feasibility Guide): https://www.environment.nsw.gov.au/resources/business/140685-cogeneration-feasibility-guide.pdf

Wikipedia (Cogeneration): https://en.wikipedia.org/wiki/Cogeneration