CHP: Efficiency right at the source

 

The basic principles of CHP should be clear to anyone who has looked at the clouds of hot steam billowing from the power plants and thought it rather wasteful.

In general, a CHP system consists of an engine to provide motive power, an electrical generator and a means of recovering heat from the exhaust gas stream. The engine runs the generator in the same way that large, conventional plants produce power, but rather than simply releasing exhaust gases to the atmosphere it extracts the heat that would normally be wasted.

A standard power-only generator is typically only 40% fuel-efficient, in large part because all of this thermal energy goes to waste.

CHP systems, however, capture and make use of this heat, driving their fuel efficiency to more than 75%, and in some cases as high as 88%. This means they can produce almost twice as much useful energy per unit of fuel burned as a conventional generator.

Versatile

Virtually any plant that requires substantial amounts of both electricity and heat can make effective use of a CHP system.

With the right specification and engineering modifications, the technology can also recycle captured heat as cooling energy using an absorption chiller. This is known as trigeneration or combined cooling, and is economical and environmentally friendly as it eliminates harmful refrigerants and reduces overall air emissions.

Additionally, there are now absorption chillers on the market that can be powered directly with the CHP exhaust, removing the need for an intermediate exhaust gas heat exchanger.

As a general rule, operators should be expecting a return on their investment within just three or four years, depending on the difference between their gas and electricity supply prices.

Cleaner and greener

While the prospect of reduced energy bills will undoubtedly be the biggest draw of a CHP system, the technology also comes with major environmental benefits that shouldn’t be overlooked.

If it is run on conventional fuel, such as natural gas, a CHP system will still produce carbon dioxide. However, because it will reduce the amount of fuel needed to run your plant, a system will be likely to cut your emissions dramatically.

According to a document produced by the UK’s Local Government Association, a well-installed and specified system will reduce the amount of carbon emitted by the site’s energy systems by a minimum of 10%, and potentially as much as 60%*.

Optimising efficiencies

If a CHP system is to operate as effectively as possible the equipment must be sized correctly, which depends on a plant’s base electrical load profile. If the system is too small, the site will have to buy in additional gas or electricity, but if it’s too big some of the heat or power it produces may be wasted.

As a site’s electricity requirement can potentially vary quite drastically throughout a year, calculating the base level of electrical consumption correctly will ensure the CHP system can withstand the day-to-day demands placed on it.

Every site will have its own ratio between how much power it needs and how much heat it needs. For example, a chemicals site may require 1300-1400kW of heat and 1300-1400kW of electricity to operate at maximum production output.

In this case the CHP system may be sized to produce 1200KWe of both heat and electricity from one natural gas fuel source, meaning that the operator only needs to import any additional power it needs above the 1200kWe base.

CHP installations are designed with this ratio in mind, and are able to meet the base load consumption during a full production year after accounting for the peaks and troughs in output.

It is also advisable to purchase an operations and maintenance contract at the same time as installation. Finning offers advice and guidance on the specification and installation of the correct CHP unit and provides an O&M contract to help ensure efficiencies are delivered as promised.

* Why Use CHP? The Local Government Association: http://www.local.gov.uk/climate-change/-/journal_content/56/10180/3510573/ARTICLE