CHP is the simultaneous generation of usable heat and power (electricity) in a single process.
The electricity is generated on or close to the site of use, allowing for capturing the resulting waste heat, therefore compared to a conventional generation system the CHP is more efficient.
This process happens in 3 main stages:
- Power generation;
- Heat recovery;
- Heat use.
The heat produced by the CHP plant can be also used to generate cooling through the installation of an absorption chiller, in which case the system is known as tri-generation.
HOW IT WORKS
A CHP plant consists of:
- a prime mover
- an electrical generator
- equipment for recovery waste heat
The ‘prime mover’ is a heat engine, which could be a gas turbine, a steam turbine or an internal combustion engine. According to the engine type the fuel to be used change. Consequently, the CHP is very flexible as it can be run with a variety of fuels like gas, oil, biogas, LPG…
The electrical generator can be found in two different forms:
- synchronous (self controlled)
- asynchronous (grid-controlled)
HOW DOES IT SAVE ENERGY?
A UK typical fossil fuel generator has an efficiency of 40 %, the remaining 60% is lost due to transmission losses through cooling towers.
A correctly sized CHP packaged system can reach an overall conversion efficiency up to 75 % through the re-use of the waste heat. This means that out of 100 units of fuel a CHP system will produce a percentage of electricity, generally 30 units and another percentage of heat around 45 units. Consequently energy savings would be around 28%-30% in comparison with a standard system.
WHEN CAN IT BE USED?
Generally, CHP systems suit projects where the demand for electricity and heating is high and extended in time. As a matter of fact, 76 % of the CHP installed capacity in the UK accounts for industrial, public and commercial sectors, of which specifically 33% is chemicals, 32% oil refineries and 10% publishing and printing.
Moreover, also public buildings such as universities, leisure centres and hospitals rely often on the combined heat ans power, as their energy loads can be highly demanding.
TYPES OF CHP SYSTEMS
There are 3 different types of CHP systems widely recognized:
- Large scale CHP– refers to large industrial application where the plant is custom-built;
- Small scale CHP– it applies to small industrial sites but also to community heating schemes, where it is supplied as packaged;
- Micro CHP- it is used in domestic and commercial developments such as care homes.
Another way to differentiate the systems is according to the output requirements, in fact custom built CHP (large scale type) have electrical power output within 1 MWe and to 100 MWe. Whether small scale varies within a range of 50 KWe and 1 MWe and the micro (small applications) provide under 50 KWe.
As a result, if a CHP system needs to be specified for a commercial or domestic building, in the early stages it might be useful looking at the demand and extended period factor together with the output required to select the most suitable system and making the most of its efficiency.
BENEFITS AND DRAWBACKS OF A CHP system
The high efficiency of a CHP plant compared with conventional bought in electricity and site-produced heat provides a number of benefits including
- On site production of power
- Reduced energy costs
- Reduction in emissions compared to conventional electrical generators and onsite boilers
The drawbacks are:
- Electricity and heat demand must be simultaneous in order for the CHP to be a viable solution (in the summer months this is often a problem);
- High maintenance cost;
- High capital cost.