Due to disruptive impacts of climate change actions are globally taken to reduce greenhouse gas (GHG) emissions associated with energy utilization to a considerable amount. Established 1997 Kyoto protocol is an international goal to attempt to alleviate impacts of climate change by reducing amount of greenhouse gas emissions into the environment. The UK has targeted to reduce its greenhouse gas emissions by 80% by the year 2050. This will be a challenging target for the UK. It is reported that over 30% of carbon dioxide emissions are from houses in the UK. This highlights the importance of employing new technologies at domestic and small scale to reduce carbon dioxide emitted from this ...view middle of the document...
If the thermal output of the CHP device cannot be fully utilised, then the CHP system cannot expect to deliver a net benefit relative to grid electricity and a highly efficient condensing boiler (Beausoleil-Morrison 2008).
CLASSIFICATION OF COMBINED HEAT AND POWER SYSTEMS
Mainly three kinds of technologies are used in CHP plants.
• Heat engine based CHP systems
• Fuel cell based CHP systems
• Solar thermal based CHP systems
Fossil fuel power stations can be integrated into CHP systems. In an engine based CHP system fuel will be burnt in the combustion chamber to create hot water or steam dependent on its engine type, and then the fluid does mechanical work resulting in electricity generation. After electricity generation, dissipated heat from the fluid can be recovered and reused. Basically sequential operation of engine based CHP system is in three stages. First electricity generation, then recovering heat, and at the end reusing of recovered heat. Sometimes the recovered heat in a CHP system will be used for cooling purposes as well as heating. This sort of a CHP system is named tri-generation. Fuel cell systems can be very efficient in converting chemical energy of hydrogen into power while a part of unused energy becomes in use as heat. Solar panels are also capable of providing heat in addition to electricity when needed. Although all of above technologies can be selected for a CHP plant, considering the quality of heat required from CHP plant will narrow the options. For instance, if a high temperature heat is required a gas-based or high temperature fuel cell CHP will be used to generate high quality waste heat. Another consideration in selection of CHP systems is when generation of electricity is more important than providing heat or vice versa. When the heat generation is a priority for instance for some industries steam producing boiler and steam turbines are the most proper selection while at a domestic level where electricity and hot water is only needed a low temperature fuel cell can be a good selection. Fig 1 shows efficiency of different CHP technologies.
Micro-CHP technologies are mainly classified into three types for power generation for an individual building: Internal combustion engine (ICE), Stirling engine (SE), and fuel cells. These technologies are different in efficiency and operation. Fig 2 shows characteristics of these three technologies. It is clearly seen in Fig 2 that fuel cells have more operational benefits such as higher performance, low heat to power ratios (H:P), quieter operation compared to other technologies.
The heat to power ratio in a CHP system is the amount of heat output per unit of electricity production. This ratio is an important CHP characteristic. A CHP system with high heat to power ratio is less desirable because electricity generation when there...