Micro combined heat and power

Micro combined heat and power or micro-CHP or mCHP is an extension of the idea of cogeneration to the single/multi family home or small office building in the range of up to 50 kW. Local generation has the potential for a higher efficiency than traditional grid-level generators since it lacks the 8-10% energy losses from transporting electricity over long distances. It also lacks the 10–15% energy losses from heat transfer in district heating networks due to the difference between the thermal energy carrier (hot water) and the colder external environment. The most common systems use natural gas as their primary energy source and emit carbon dioxide. Micro combined heat and power or micro-CHP or mCHP is an extension of the idea of cogeneration to the single/multi family home or small office building in the range of up to 50 kW. Local generation has the potential for a higher efficiency than traditional grid-level generators since it lacks the 8-10% energy losses from transporting electricity over long distances. It also lacks the 10–15% energy losses from heat transfer in district heating networks due to the difference between the thermal energy carrier (hot water) and the colder external environment. The most common systems use natural gas as their primary energy source and emit carbon dioxide. Combined heat and power (CHP) systems for homes or small commercial buildings are often fueled by natural gas to produce electricity and heat. A micro-CHP system usually contains a small fuel cell or a heat engine as a prime mover used to rotate a generator which provides electric power, while simultaneously utilizing the waste heat from the prime mover for an individual building's heating, ventilation, and air conditioning. A micro-CHP generator may primarily follow heat demand, delivering electricity as the by-product, or may follow electrical demand to generate electricity and use heat as the by-product. When used primarily for heating, micro-CHP systems may generate more electricity than is instantaneously being demanded in circumstances of fluctuating electrical demand. The heat engine version is a small scale example of cogeneration schemes which have been used with large electric power plants. The purpose of cogeneration is to utilize more of the energy in the fuel. The reason for using such systems is that heat engines, such as steam power plants which generate the electric power needed for modern life by burning fuel, are not very efficient. Due to Carnot's theorem, a heat engine cannot be 100% efficient; it cannot convert anywhere near all the heat produced from the fuel it burns into organized forms of energy such as electricity. Therefore, heat engines always produce a surplus of low-temperature waste heat, called 'secondary heat' or 'low-grade heat'. Modern plants are limited to efficiencies of about 33–63% at most, so 37–67% of the energy is exhausted as waste heat. In the past this energy was usually wasted to the environment. Cogeneration systems, built in recent years in cold-climate countries, utilize the waste heat produced by large power plants for heating by piping hot water from the plant into buildings in the surrounding community. However, it is not practical to transport heat long distances due to heat loss from the pipes. Since electricity can be transported practically, it is more efficient to generate the electricity near where the waste heat can be used. So in a 'micro-combined heat and power system' (micro-CHP), small power plants are instead located where the secondary heat can be used, in individual buildings. Micro-CHP is defined by the EC as being of less than 50 kW electrical power output, however, others have more restrictive definitions, all the way down to <5 kWe. In centralized power plants, the supply of 'waste heat' may exceed the local heat demand. In such cases, if it is not desirable to reduce the power production, the excess waste heat must be disposed of (e.g. cooling towers or sea cooling) without being used. A way to avoid excess waste heat is to reduce the fuel input to the CHP plant, reducing both the heat and power output to balance the heat demand. In doing this, the power production is limited by the heat demand. In a traditional power plant delivering electricity to consumers, about 34.4% of the heat content of the primary heat energy source, such as biomass, coal, solar thermal, natural gas, petroleum or uranium, reaches the consumer, although the efficiency can be 20% for very old plants and 45% for newer gas plants. In contrast, a CHP system converts 15%–42% of the primary heat to electricity, and most of the remaining heat is captured for hot water or space heating. In total, over 90% of the heat from the primary energy source (LHV based) can be used when heat production does not exceed the thermal demand. CHP systems are able to increase the total energy utilization of primary energy sources, such as fuel and concentrated solar thermal energy. Thus CHP has been steadily gaining popularity in all sectors of the energy economy, due to the increased costs of electricity and fuel, particularly fossil fuels, and due to environmental concerns, particularly climate change. CHP systems have benefited the industrial sector since the beginning of the industrial revolution. For three decades, these larger CHP systems were more economically justifiable than micro-CHP, due to the economy of scale. After the year 2000, micro-CHP has become cost effective in many markets around the world, due to rising energy costs. The development of micro-CHP systems has also been facilitated by recent technological developments of small heat engines. This includes improved performance and cost-effectiveness of fuel cells, Stirling engines, steam engines, gas turbines, diesel engines and Otto engines. PEMFC fuel cell mCHP operates at low temperature (50 to 100 °C) and needs high purity hydrogen, its prone to contamination, changes are made to operate at higher temperatures and improvements on the fuel reformer. SOFC fuel cell mCHP operates at a high temperature (500 to 1,000 °C) and can handle different energy sources well but the high temperature requires expensive materials to handle the temperature, changes are made to operate at a lower temperature. Because of the higher temperature SOFC in general has a longer start-up time and need continuous heat output even in times when there is no thermal demand.