Selecting CHP Plant

Once the energy and cost data have been collected and tabulated, the next stage of the initial feasibility study is to select a potentially suitable CHP system.

As a minimum, information obtained should include:

• Electrical output, which should include data relating to the power consumption of the CHP plant’s own motors etc., so that the net output can be defined.
• Heat output that can be recovered for use on-site, including data on the temperature and flow rate of the fluid in which the heat is contained.
• Fuel consumption of the equipment, taking care to ensure that this can be expressed in gross calorific value terms.
• The cost of supplying and installing the equipment.
• The dimensions and weight of the equipment.
• The approximate cost per kilowatt hour (kWh) generated that should be allowed for servicing and maintaining the equipment.
• Any essential auxiliary items that are not contained within the scope of the equipment.
• After-sales service, including on-site maintenance provision, availability of parts etc.

Selection of prime movers for cogeneration

Steam turbines may be the appropriate choice for sites where:

• electrical base load is over 250 kWe
• there is a high process steam requirement; and the heat to power demand ratio is greater than 3:1
• cheap, low-premium fuel is available
• adequate plot space is available
• high grade process waste heat is available (e.g. from furnaces or incinerators)
• existing boiler plant is in need of replacement
• heat to power ratio is to be minimised, using a gas turbine combined cycle

Gas turbines may be suitable if:

• power demand is continuous, and is over 1 MWe (smaller gas turbines are just starting to penetrate the market)
• natural gas is available (although this is not a limiting factor)
• there is high demand for medium/high pressure steam or hot water, particularly at temperature higher than 140°C
• demand exists for hot gases at 450°C or above – the exhaust gas can be diluted with ambient air to cool it, or put through an air heat exchanger (Also consider using in a combined cycle with a steam turbine)
  

Reciprocating engines may be suitable for sites where:

• power, or processes are cyclical or not continuous
• low pressure steam or medium or low temperature hot water are required
• there is a low heat to power demand ratio
• when natural gas is available, gas powered reciprocating engines are preferred
• when natural gas is not available, fuel oil or LPG powered diesel engines may be suitable
• electrical load is less than 1 MWe - spark ignition (units available from 3 kWe to 10 MWe)
• electrical load greater than 1 MWe - compression ignition (units from 100 kWe to 20 MWe)

Identifying CHP plant of an appropriate output

Initial selection of CHP plant is often dictated by two factors:

The site heat demand, in terms of quantity, temperature etc., that can be met using heat from the CHP plant.

The base-load electrical demand of the site, i.e. the level below which the site electrical demand seldom falls.

Sizing on heat demand will maximise energy and environmental savings. Depending on the heat to power ratio of site energy demands, sizing to match the heat requirement will result in a scheme that may offer a surplus of electricity generation (eg during the night) or may require top-up electricity supplies (eg at times of peak electricity demand). The economics of exporting the electricity then becomes a key issue in determining economic CHP plant size.

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© Irish CHP Association. Last Updated: Fri 13 May 2005.