Micro-CHP

The principle of the combined heat and power generation (CHP) or cogeneration is improved fuel efficiency by producing heat and electricity simultaneously. The same amount of fuel generates more energy, and less energy is lost in comparison to conventional power plants, since the heat generated when fuel is burnt to produce electricity is captured and utilised for some useful purpose such as space heating, water heating or refrigeration.

Due to the improved energy efficiency, CHP helps to avoid CO 2 emissions, because the excess heat of electricity generation is directly used. In conventional power plants about 35% of the energy potential contained in the fuel is converted into electricity, whilst the rest is lost as waste heat. Even the most advanced technologies do not convert more than 55 % of fuel into useful energy. In comparison, cogeneration is able to achieve energy efficiency from about 90 % meaning that only circa 10 % of the used fuel is transformed into heat loss.

38 % Power

Combined Heat and Power Generation

50% Heat

38 % Power

160 % fuel

6% loss

losses

78 % total losses

Seperate Heat and Power Generation

100 % fuel

50 % Heat

12 % losses

Source: BKWK

Less use of primary energy implies also less emission of CO 2 . By using CHP, CO 2 emissions are reduced about 34% compared to the conventional generation of heat and power.

The advantages of CHP are obvious. That is why the European Union and its member states are willing to rise the percentage of CHP in electricity and heat production by a notable amount in the next years.

Cogeneration units have different sizes, ranging from an electrical capacity of less than 5 kWe (e.g. for a single-family house) up to 500 MWe (e.g. district heating or industrial cogeneration). Small scale units are most qualified sited close to the heat and power demand and, ideally, are built to meet this demand as efficiently as possible. In this decentralised generation, often more electricity is generated than is needed by the owner himself. The surplus electricity can be sold to the local grid operator or supplied to another customer via the net distribution system.

Small or Micro CHP are units which reach an electric power output up to 50 kWe (according to European Directive 2004/8/EG). The generation units are sited in close vicinity to the user where the heat is needed, because this reduces line losses to a minimum and puts operators in a position to open up economic profits for themselves. A CHP station consists of a CHP unit and a heating boiler to compensate peaks in the energy consumption on very cold days or to compensate blackout or technical service.

CHP is deployable manifoldly. Hotels, restaurants, schools, hospitals, housing or public buildings are using CHP already today. It can be used, wherever there is need for both electricity and heat. Each owner has to assess his needs for heat and power consumption to implement the right size of CHP for his individual energy consumption to run the CHP economically. CHP systems can, with the addition of a chiller, supply cooling for air conditioning systems as well as heating - such an arrangement is often called a ‘trigeneration' system.

Source: Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU)

A range of technologies can be applied to cogenerate electricity and heat. All cogeneration schemes will always include an electricity generator and a system to recover the heat. The most known technologies are steam turbines, gas turbines, combined cycle (gas and steam turbines), Diesel and Otto Engines. These technologies are readily available and approved. Three other technologies have recently appeared on the market, or are likely to be commercialised within the next few years: Micro-turbines, fuel cells and stirling engines, mostly used for micro CHP.

•  Diesel or gas engines have a standard engine driving an alternator to convert mechanical work produced at the engine shaft into electricity. The heat of exhaust gases, i.e. heat resulting form combustion during power generation, is used for process heat supply.

•  Micro-turbines have small capacity between 1 and 250 KWe. The gas is burned in an external combustion chamber fed in pressurised air from a compressor. The flue gas produced is led into a turbine, where the chemical energy is partly converted into mechanical energy, which drives the alternator. The thermal energy remaining in the flue gas at turbine outlet can be used in a heat exchanger to obtain process heat, i.e. steam or hot water.

•  An alternative for small-scale electricity production is the Stirling engine. It is based on a closed cycle, where a working gas is alternately compressed in a cold cylinder volume and expanded in a hot cylinder volume. The heat is transferred form the outside through a heat exchanger in the same way as in a steam boiler. Therefore, the engine is comparable to the biomass combustion technology.

•  In a steam turbine, where a gasifier or direct combustion is combined with a steam engine, mechanical energy is produced by the expansion of high-pressure steam. The heat is recovered at the exit of the engine. Flue gas, gas resulting form combustion passes through a boiler in which steam is generated. The steam flows into the steam engine where by expansion it is performing mechanical work that is later converted into electrical energy in the generator. After this, steam passes into the condenser where incidental condensation heat can be used as district or process heat. The water is brought to operation pressure by a feed water pump and then is fed to a boiler, thus closing the cycle.

CHP systems can be used with nearly every fuel: Either with fossil fuels such as coal, lignite, natural gas as well as oil or with renewable energies such as biogas, vegetable oil, pellets, wood or hydrogen. When using the same fuel, CHP is always superior to conventional power and heat generation in terms of energy savings and reduction of CO 2 emissions.