Micro combined heat and power, micro-CHP, μCHP 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. Usual technologies for the production of heat and power in one common process are e.g. internal combustion engines, micro gas turbines, stirling engines or fuel cells.
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 transport in 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; nevertheless the effective efficiency of CHP heat production is much higher than of a condensing boiler, and thus reducing emissions and fuel costs.
Overview A micro-CHP system usually contains a small 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 space heating and the provision of hot domestic water. With fuel cells there is no rotating machinery, but the fuel cell's stack and where applicable also the reformer will provide useful heat. The stack does generate DC power which is converted by DC/AC inverter into mains voltage. Micro-CHP is defined by the EU as less than 50 kW electrical power output, however, others have more restrictive definitions, all the way down to <5 kWe.
A micro-CHP generator may primarily follow heat demand, delivering electricity as the by-product, or may follow electrical demand to generate electricity, with heat as the by-product. When used primarily for heating, micro-CHP systems may generate more electricity than is instantaneously being demanded; the surplus is then fed into the grid.
The purpose of cogeneration is to make use of more of the chemical energy in the fuel. The reason for using CHP systems is that large thermal power plants which generate electric power by burning fuel produce between 40% and 60% low-temperature waste heat, due to Carnot's theorem. The temperature produced by this waste heat (around 80 °C - 150 °C) does allow it to be used for space heating purposes, therefore in some urban areas district heating networks have been installed. Heat networks have a limited extent, as it is not economical to transport heat long distances due to heat loss from the pipes, and it will not reach into areas of low population density, or else revenues per CAPEX will go down. Where no district heating is possible due to low heat demand density or because the local utility has not invested in costly heat networks, this thermal energy is usually wasted via cooling towers or discharged into rivers, lakes or the sea.
Micro CHP systems allow highly efficient cogeneration while using the waste heat even if the served heat load is rather low. This allows cogeneration to be used outside population centers, or even if there is no district heating network. It is efficient to generate the electricity near the place where the heat can also be used. Small power plants (μCHP) are located in individual buildings, where the heat can be used to support the heating system and recharge the hot domestic water tank, thus saving heating oil or heating gas. CHP systems are able to increase the total energy utilization of primary energy sources. 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.
In a traditional power plant delivering electricity to consumers, about 34.4% of the primary energy of the input fuel, such as coal, natural gas, uranium, petroleum, solar thermal, or biomass, reaches the consumer via electricity, 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.
Since 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.
Combined heat and power (CHP) systems for homes or small commercial buildings are usually fueled by natural gas to produce electricity and heat. If no access to the natural gas network is available, which in general is the cheapest alternative, LPG, LNG or heating fuel (diesel) might be an alternative. The PEMFC fuel cell mCHP operates at low temperatures (50 to 100 °C) and needs high purity hydrogen. It is prone to contamination; changes are made to operate at higher temperatures and improvements on the fuel reformer. The SOFC fuel cell mCHP operates at a high temperature (500 to 1,000 °C) and can handle different fuel sources well, but the high temperature requires expensive materials to handle it; changes are made to operate at a lower temperature. Because of the higher temperature the SOFC in general has a longer start-up time and needs continuous heat output even at times when there is no thermal demand.
CHP systems linked to absorption chillers can use waste heat for refrigeration.Tri-Generation success story
A 2013 UK report from Ecuity Consulting stated that MCHP is the most cost-effective method of utilizing gas to generate energy at the domestic level.The role of micro CHP in a smart energy world Micro CHP report powers heated discussion about UK energy future
The fuel cell industry review stated in 2013 that with 64% of global sales the fuel cell micro-combined heat and power had passed the conventional engine-based micro-CHP systems in sales in 2012.The fuel cell industry review 2013
Technologies Micro-CHP engine systems are currently based on several different technologies: Internal combustion engines Stirling engines Fuel cell Microturbines Steam engine/Steam motor (using either the traditional water or organic chemicals such as refrigerants)
Fuels There are many types of fuels and sources of heat that may be considered for micro-CHP. The properties of these sources vary in terms of system cost, heat cost, environmental effects, convenience, ease of transportation and storage, system maintenance, and system life. Some of the heat sources and fuels that are being considered for use with micro-CHP include: natural gas, LPG, biomass, vegetable oil (such as rapeseed oil), woodgas, solar thermal, and lately also hydrogen, as well as multi-fuel systems. The energy sources with the lowest emissions of particulates and net-carbon dioxide include solar power, hydrogen, biomass (with two-stage gasification into biogas), and natural gas. Due to the high efficiency of the CHP process, cogeneration has still lower carbon emissions compared to energy transformation in fossil driven boilers or thermal power plants.
The majority of cogeneration systems use natural gas for fuel, because natural gas burns easily and cleanly, it can be inexpensive, it is available in most areas and is easily transported through pipelines which already exist for over 60 million homes.
Engine types Reciprocating internal combustion engines are the most popular type of engine used in micro-CHP systems. Reciprocating internal combustion engine based systems can be sized such that the engine operates at a single fixed speed, usually resulting in a higher electrical or total efficiency. However, since reciprocating internal combustion engines have the ability to modulate their power output by changing their operating speed and fuel input, micro-CHP systems based on these engines can have varying electrical and thermal output designed to meet changing demand.
Natural gas is suitable for internal combustion engines, such as Otto engine and gas turbine systems. Gas turbines are used in many small systems due to their high efficiency, small size, clean combustion, durability and low maintenance requirements. Gas turbines designed with foil bearings and air-cooling operate without lubricating oil or coolants. The waste heat of gas turbines is mostly in the exhaust, whereas the waste heat of reciprocating internal combustion engines is split between the exhaust and cooling system.
External combustion engines can run on any high-temperature heat source. These engines include the Stirling engine, hot "gas" turbocharger, and the steam engine. Both range from 10%-20% efficiency, and as of 2014, small quantities are in production for micro-CHP products.
Other possibilities include the Organic Rankine cycle, which operates at lower temperatures and pressures using low-grade heat sources. The primary advantage to this is that the equipment is essentially an air-conditioning or refrigeration unit operating as an engine, whereby the piping and other components need not be designed for extreme temperatures and pressures, reducing cost and complexity. Electrical efficiency suffers, but it is presumed that such a system would be utilizing waste heat or a heat source such as a wood stove or gas boiler that would exist anyway for purposes of space heating.
The future of combined heat and power, particularly for homes and small businesses, will continue to be affected by the price of fuel, including natural gas. As fuel prices continue to climb, this will make the economics more favorable for energy conservation measures, and more efficient energy use, including CHP and micro-CHP.
Fuel cells Fuel cells generate electricity and heat as a by product. The advantages for a stationary fuel cell application over stirling CHP are no moving parts, less maintenance, and quieter operation. The surplus electricity can be delivered back to the grid.Integration of fuel cell micro-CHPs on low voltage grid: A Danish case study
PEMFC fuel cells fueled by natural gas or propane use a steam reformer to convert methane in the gas supply into carbon dioxide and hydrogen; the hydrogen then reacts with oxygen in the fuel cell to produce electricity. A PEMFC fuel cell based micro-CHP has an electrical efficiency of 37% LHV and 33% HHV and a heat recovery efficiency of 52% LHV and 47% HHV with a service life of 40,000 hours or 4000 start/stop cycles which is equal to 10 year use. An estimated 138,000 Fuel cell CHP systems below 1 kW had been installed in Japan by the end of 2014. Most of these CHP systems are PEMFC based (85%) and the remaining are SOFC systems.
In 2013 Lifetime is around 60,000 hours. For PEM fuel cell units, which shut down at night, this equates to an estimated lifetime of between ten and fifteen years.Latest developments in the Ene-Farm scheme
United States Department of Energy (DOE) Technical Targets: 1–10 kW residential combined heat and power fuel cells operating on natural gas.DOE Distributed/Stationary fuel cell systems +Development of fuel cellsType 2008 2012 2015 2020Electrical efficiency at rated power2 34%40%42.5%45%CHP energy efficiency3 80%85%87.5%90%Factory cost4 $750/kW$650/kW$550/kW $450/kWTransient response (10%–90% rated power) 5 min 4 min 3 min 2 minStart-up time from 20 °C ambient temperature60 min 45 min 30 min 20 minDegradation with cycling5< 2%/1000 h 0.7%/1000 h0.5%/1000 h 0.3%/1000 hOperating lifetime66,000 h 30,000 h40,000 h60,000 hSystem availability97%97.5%98%99% 1 Standard utility natural gas delivered at typical residential distribution line pressures.