You are here: Micro CHP?
Continue to: English
Common:
A: Micro Combined Heat and Power (Micro CHP):
CHP is a form of cogeneration, generating electricity and heat simultaneously, allowing for higher efficiencies compared to using two separate systems. On larger scale, CHP systems are used to increase efficiencies in thermal power plants (nuclear or fossil fuels), by capturing waste heat from exhaust gases and recycling it. The primary objective for these is to generate electricity, heat being a useful by-product.
On the contrary, micro CHP systems primary objective is to generate heat, meaning the amount of electricity generated is dependant upon the heat required. These systems use one of the following technologies: • Reciprocating Engines: a piston engine, such as a combustion engine in cars. • Stirling Engines: a heat engine containing a sealed circuit filled with a working gas such as helium, hydrogen or even air. The system contains 2 pistons, each with a heat exchanger; one hot and one cool (though the cool can just be air). The greater the temperature difference, the higher the efficiency. • Steam Engine: water is heated and as the steam heats up it expands, and this can be used to create mechanical movement. • Rankine Cycle Engine: an engine that follows the Rankine Cycle as close as possible (to reach a high efficiency). • Fuel Cells: rather like a battery, they are an electrochemical energy conversion unit with a supply of reagents. A typical and practical example is the hydrogen fuel cell, which uses hydrogen and oxygen.
Domestic systems are designed to run off natural gas, though it is possible to use other fuels. All are still in their early stages of R&D, though the Stirling Engine and Fuel Cell CHP systems are becoming close to the commercial market.
Micro CHP has had interest in the UK recently, with the carbon trust currently running a trial on small-scale (>25kW) and micro (around 1kW rating) CHP units. The trial started in 2003, with the goal of collecting independent data on performance/saving benefits, and included Stirling engines, Organic Rankine Cycle machines, fuel cells and internal combustion engines.
Data is currently being generated from the 40 units within the trial, which points to the potential of the systems but also that there is still much need for development. A third of systems achieve reduced emissions, another third with little/no difference and the final third with increased emissions. Small CHP systems appeared to be more efficient than the micro CHP, but this could quite possibly be due to more R&D (data from „The Carbon Trust’s Micro and Small-Scale CHP Field Trial Update Report“, November 2005. The studies final conclusions are due to be published in the summer, 2007).