What is Co-Generation?

Co-generation (or CHP) is the production of both electricity and heat. Locally available fuel can be burned to produce elctricity, however the process is relatively inefficient (~25-30%). Utilising the surplus heat produced from this may increase the efficiency of co-generation to 70% or higher. The economic viability of co-generation will be dependent on the price difference between fuel available for co-generation and cost of electricity from the grid. Where the fuel available is effectively free or has a negative cost associated with it, for instance in the case of by-products or waste streams from industrial processes, co-generation may be an attractive option. Rising costs of grid electricity will also improve the economic viability of local co-generation schemes.

Main Advantages

• Increased Conversion Efficiency. Using heat and power increases efficiency dramatically.
• Reduced transport costs. Local use of available fuel (for instance fuel produced from industrial byproducts) can cut down on transport costs and associated CO2 emmissions.
• Reduced Transmission Losses. Localised generation can avoid transmission losses associated with centralised generation.
• Reduced energy costs as localised co-generation can displace imported energy (e.g. grid electricity and natural gas or fuel oil used in boilers).
• Reduction in CO2 – at least 10% reduction in CO2 emissions compared to other forms of electricity generation
• Adaption of current technologies – the technologies used for CHP; reciprocating engines, steam and gas turbines are established technologies that have been adapted for use in co-generation.
• Flexible supply suited to user's needs. – CHP can work in a number of modes; supplying a constant load or following the electrical or the thermal demand dependent on individual user's best option.

Co-generation Plant

There are a number of ways of processing the product to produce heat and electricity. The different processes all have their own advantages and disadvantages and should all be considered for present and future projects.

Internal Combustion Engine

Currently internal combustion engines operate in most small scale co-generation units, with Stirling engines being used in very small scale applications. The combustion of the biogas has such a high temperature and pressure that it is able to drive the piston or turbine blades in the engine. This will provide mechanical energy to the generator linked to the engine in order to produce electricity.

The ICE has a reasonably high electrical efficiency as well as a high overall efficiency. It also has a large range of power capacities and is able to run from an assortment of fuels. Problems with the ICE are that the many moving parts mean regular maintenance is needed and it can be rather noisy.

Stirling Engine

A Stirling Engine is a reciprocating engine with a closed cylinder so the combustion takes place outside the cylinder. The piston is able to move in the cylinder because of the compression and expansion of the gas and this is due to the heating and cooling of the cylinder from external combustion. The benefit is that there are low emissions, especially NOx and lower noise production. The external combustion also decreases the servicing needs and allows various fuels to be used. Unfortunately there is a low electrical efficiency when natural gas is used. There are better for smaller electrical loads (2-50kW) and have an electricity/heat ratio appropriate for residential buildings.

Gas Turbine

In large scale units it is mostly gas turbines that are used with microturbines becoming available in the past 20 years. They are best here due to their power/thrust to size/weight ratio.

The gas flows into a rotary engine and the energy is extracted from it to the gas turbine. The gas flows through the compressor via a combustion chamber to the turbine . They can be reasonably efficient, up to 60% and the waste heat from the exhaust can be recovered and used for space and water heating. An advantage of these engines is that they can readily be turned off and on so that a varying peak demand can be supplied. However they are less efficient than the IC engine in terms of the work out and the fuel used and tend to have a lower electrical efficiency than the IC engines.

Micro-Turbine

Usually classed as a gas turbine that can generate power in the range 25 to 250kW. In general there are made up of a generator, compressor, combustion chamber and turbine. The heat of the exhaust gas has a high temperature range which is very good for the heat recovery and production of energy so this turbine is better for processes with high temperatures.

The disadvantages are that there is a low electrical efficiency especially on a part load and it is inflexible to load changes. The price of the technology is still high as are the maintenance costs.

Steam Turbine

Steam turbines are also used in the larger scale systems that range from 200kWe upwards.

It is device capable of extracting thermal energy from pressurized steam and then change it into mechanical work. The electricity is generated through the Rankine Cycle. The high pressure steam turns the turbine blades and the rotor takes the power from the spinning blades and uses this in the generator. They are better in large thermal power plants and have a high thermal efficiency. They have less moving parts so maintenance costs are lower and the reliability increases.

The disadvantage is that this high efficiency can only really be reached at high speeds.

Fuel Cell

The fuel cell is a new development for use in cogeneration systems and currently is not entirely commercially available.

A fuel cell is designed to electrochemically produce energy by combining hydrogen and oxygen. It consists, in its basic form of an anode, cathode and electrolyte. The hydrogen releases the electrons on the anode which then are conducted over to the cathode and converted to water. The reaction can be intensified by heat.

They have a high electrical efficiency and low emission rates. Other advantages are that they are noiseless, reliable and have the ability to rapidly adjust to load changes. The disadvantage is that they currently have a high investment cost as they have not been fully developed and are not yet commercially available.